What's the best way to join the Army as a 17C?

Interesting -- as a career recruiter, former recruiter, now staff NCO, I will state the following -- it seems the local recruiter may not be aware of current USAREC messages or MILPER messages which state "specific" instructions for MOS applications --- I have done this before in the past with certain recruits who were interested in CID, or Linguist in which the Army was looking for. It seems the Army is currently looking for MOS 17C candidates but I believe you have to be a PVT to SGT currently on active duty. Of course the appropriate messages will specify a check list to follow -- Good luck!
ATTENTION:
MILPER MESSAGE 15-384 *EFFECTIVE 02 DEC 15*
-THERE ARE NO WAIVERS FOR THE RANK REQUIREMENT. ONLY SGTs AND SPCs CAN APPLY FOR MOS 17C.
-THE 17C POSTURE SHEET WILL BE POSTED MONTHLY. THE CURRENT POSTURE SHEET IS VALID AS OF 01 DEC 15.
-WHEN APPLYING, USE THE UPDATED 17C PACKET. WE WILL NOT ACCEPT THE OLD PACKET DOCUMENTS.

Up until recently the Army was transitioning Soldiers from 35Q to 17C. It was mostly random with little rhyme or reason. Since then you have to put in a 4187 with additional documents to request transitioning to 17C. If your brother-in-law is still considering it, he will be best to go 35Q and put in a 4187 after a year in Fort Meade/Gordon (likely initial assignments). He MAY possibly get either Schofield Barracks or Joint Base San Antonio, but those are smaller detachments and not as likely for him to go there.

I would recommend 35Q since there is no 17C school... yet. Most of the 17c were switched from 35Q, so they will pretty much have the same knowledge/skill set coming out. Both will do some type of cyber related job either way.