Where can a single mom go for financial help?

Since you enrolled him into DEERS, you need to do a DA form 5960 to change your BAH to start getting BAH with dependent rate. (Google "Army APD" and it has all the forms and all regulations as well). Then route that through your company to S1 so you can start to get BAH and BAS.

Do you mean you pay your grandma who provides child care like money for child care?

Then you need to do a Family Care Plan (AR 600-20, Chapter 5). Read that and it tells you all the forms you need to fill out. These are the key documents but go through and read that chapter so you know everything about FCP:

(2) DA Form 5305 (Family Care Plan) is not a legal document that can change a court-mandated custodial arrangement, nor can it interfere with a parent’s right to custody of their child. Its sole purpose is to document for Army purposes the plan by which Soldiers will provide for the care of their Family members when military or civilian duties prevent them from doing so. It will include proof that guardians and escorts have been thoroughly briefed on the responsibilities they will assume and the procedures for accessing military and civilian facilities and services on behalf of the Family members of the Soldier. It will attest that the guardian and escort agreed to provide care and have been
provided all necessary legal authority and means to do so. It will include proof that the Soldier has obtained consent to the planned designation of guardianship from all parties with a legal interest in the custody and care of the minor child, or proof that reasonable efforts have been made to obtain consent to such designation.
(3) At a minimum, proof will consist of the following attachments to DA Form 5305:
(a) DA Form 5841 (Power of Attorney) or equivalent delegation of legal control.
(b) DA Form 5840 (Certificate of Acceptance as Guardian or Escort).
(c) DD Form 1172–2 (Application for Identification Card/DEERS Enrollment) for each Family member.

Now you should definitely have a plan for short term care (staff duty for example) but the FCP is primarily for say TDY, deployment, etc. I'm a single parent and have been for 7 years now, so feel free to message me with any questions.

You will need several things. You will need a Family Care Plan. It is required by regulations for single parents. You will need to initiate a DA Form 5960 and push that up to your S1.

Look into AR 608-99. You are authorized to receive BAH and BAS and live in housing, whether on or off post. You are obligated to have a home for your child to live in, even if he lives with your grandma.