How do I break a Soldier's habit of lying?

SGT(P) (Join to see) Habitual liars are very difficult to break from their constant lying. My guess is this SPC has lied continuously his whole life, maybe for attention seeking, a way to fit in or whatever other reason. Most likely you are not going to change him. The constant lying is a way of life and at this point he may not even realize what a fool he is making of himself because I am pretty sure you are not the only one who knows everything that comes out of his mouth is a lie. Maybe (probably not though) you could get him to go seek psychiatric help.

SGT(P) (Join to see)

After what you have tried, id take the last step.
Print and present the stuff below.

Sit the SM down and counsel. Each lie will generate a negative counseling session and recommendation for UCMJ, once sufficient evidence is accumulated a request for involuntary separation under AR 635-200 chapter 14 will be made to the commander.

Change as of NOW, or start planning for your separation.

SEPARATION FOR MISCONDUCT, CHAPTER 14, AR 635-200
Note: This handout only provides basic information and is not intended to serve
as a substitute for a personal consultation with an attorney.
BASIS FOR THIS CHAPTER
a. This discharge may be given to a Soldier upon the following:
(1) The Soldier's conduct exhibits a pattern of minor military disciplinary
infractions; or
(2) The Soldier's conduct exhibits a pattern of discreditable involvement
with civilian authorities or a pattern of conduct prejudicial to good order and discipline
(paragraph 14-12b); or
(3) The Soldier commits a single act of "serious misconduct" (paragraph
14-12c) "serious misconduct" is an offense that could be punished by a punitive
discharge under the UCMJ.
WHAT MUST THE COMMANDER DO BEFORE THEY CAN INITIATE A CHAPTER
14?
a. Only for discharge under paragraphs 14-12a and 14-12b, the Soldier must be
formally counseled in writing at least once prior to initiation of the chapter action. The
counseling must include the fact that separation action may be initiated if the behavior
continues, the type of discharge that could result from the possible separation action
and the effect of each. The Soldier is given a reasonable time to correct his deficiencies
(Para 1-18b, AR 635-200).
b. Again, only for discharge under paragraphs 14-12a and 14-12b, the Soldier
must be transferred once with at least two months of duty in each unit, between
battalion size or larger units. The separation authority can waive this transfer
requirement if further duty by the Soldier would cause disciplinary problems or if the
member is resisting rehabilitative efforts.
c. Discharge under paragraph 14-12c does not require the above rehabilitation
and counseling.
2
d. The Soldier must be given a proper medical examination and mental status
evaluation.
PROCEDURE
The unit commander forwards the chapter packet containing all relevant matters,
including any statements from you and documents you submitted, to the Battalion
Commander, who makes his recommendation and forwards it to the Brigade
Commander who makes the final decision as to whether you should be separated and
the type of discharge you should receive. All records of this separation action, including
the reason for separation, are confidential by operation of Federal Law. The only thing,
which will be known by prospective employers, etc. is the type of discharge that you
received (i.e., Honorable, etc.).

WHAT ARE YOUR RIGHTS?
a. You have a right to consult with an attorney within a reasonable time (not less
than three duty days) or civilian counsel at your own expense (your command should
make a TDS appointment for you).
b. You have the right to submit statements in your own behalf (these statements
may be made by you or by others on your behalf).
c. You have the right to obtain copies of documents that your commander will
forward in support of the separation recommendation.
d. You can waive the above rights in writing. Failure to respond (that means not
signing the form) within seven duty days from the date you see an attorney means you
give up all of your rights; and
e. You have the right to an administrative discharge board if you are considered
for discharge with an Under Other Than Honorable Conditions (UOTH) characterization
of service or if you have six (6) or more years of service of total active and reserve
service on the date of initiation of recommendation for separation.

TYPE OF DISCHARGE
Soldiers separated under this program will generally be given either a general under
honorable conditions or an under other than honorable conditions discharge (Para. 3-7,
AR 635-200). They may also, but rarely, receive an Honorable discharge.
a. HONORABLE: This is a separation with honor. An honorable discharge will
be given for proper military behavior and proficient performance of duty. If the Soldier
has served faithfully and performed to the best of his ability, and there is no derogatory
information in his military record, he should receive an honorable discharge. If there are
3 infractions of discipline, the seriousness and frequency will be evaluated. When there is a doubt, it should be resolved in favor of an honorable discharge.
b. GENERAL: This is a discharge under honorable conditions, Soldier whose
record and performance is satisfactory. This is usually given to someone who had nonjudicial punishments (Article 15s), but not for serious infractions. When this type of
discharge is given, the commander must state the specific basis for it.
c. UNDER OTHER THAN HONORABLE CONDITIONS: This is a discharge
issue ordinarily for misconduct, fraudulent entry, security reasons, or for the good of the service. This discharge carries a considerable stigma and deprives the Soldier of
substantially all veteran’s benefits accrued during the enlistment period.

And print excerpts from, Army Regulation 635–200 chapter 14

Therapy. He may have grown up in an environment where he had to lie to avoid punishments that didn’t fit his “crime”. There’s a reason why they’re lying.