If your Dad is that much of a hardass, he sounds like the kind of guy who would definitely side with you on this one. Explain calmly, rationally that you believe this younger kid is gay and has been trying to touch you a lot. You didn't want to be a rat, so you let it slide for a while. But when he grabbed your ass, it was the final straw.
Most father's biggest fears (or one of them) is that their son will turn out gay. If you explain that you are trying to stop gay advances toward you, he should be with you 100%. Once you can get your dad on your side, the pricinpal will cave like a cheap card table. Schools are simply terrified of lawsuits (not that it would come to that).
If all else fails and you still get max punishment, I'd tell your parents and your assistant principal that since they've advocated it by their actions, you will begin grabbing plenty of chick's asses from now on. After all, it's not a punishable offense according to the way they've handled the situation, right?