I would check inside of his mouth, Kyle... I have never seen a garter that was unable to flick it's tongue at will unless it was dehydrated or something was going on inside it's mouth... even if they had yet to shed their tongue. I don't see how that would possibly stop him from flicking it. That skin is so thin and fine, I don't see how just that would stop him from flicking. If he has an infection of some sort starting in his mouth, it could be getting sticky and a buildup of sticky saliva or something else can be holding his mouth shut and making him unable to flick his tongue. I have seen this first hand. If you look closely at a snake that has this problem, you can see the muscles underneath his jaw contracting and it's lips curling slightly as they attempt to flick it. If a tongue stops flicking, I take it seriously. Pick him up and try to get him agitated enough to where he would HAVE to start flicking his tongue.