My starting freshman PG went ineligible with his final grades. Hes got promised start/15 MPG and is sitting at 17.7 minutes. He'll be fine with the amount of starts, but my question is: Will missing his final 5+ games due to eligibility mean that hes not going to reach his target minutes and thus I'll lose him in the off season?

You'll be fine from the point of view about your promises for starts and minutes.(26 games x 15 = 390 total minutes which he already has and they usually are happy with 80% depending on how good of a recruit they were in comparison to your prestige/level).

He obviously won't be available for the rest of the regular season and the post season but he shouldn't transfer based on missed starts/minutes.

the ineligible games do not count for or against a promise. if you are set and then you lose the dude on final grades, you remain set. if you are in trouble, you remain in trouble (but often its worse because you lost your chance to fix things).

in your case, you aren't even close - nothing to worry about.