Sounds like you're boned. There's a few things you can do.
Current situation and options
First of all, find out where is his new army coming from. Are you allowing his retreating forces to regroup, or is he hiring mercs? Find out how many armies he can still raise. You could ignore the holdings for now, and destroy his routing troups, and then crush any newly raised forces before he can doomstack.
Be prepared for a long struggle – basically, you need to use your main army to pounce on any small (or morale-deficient) enemy armies, while a minimal portion of your army liberates territory one county of the time.
Keep an eye on his cashflow, though. If he's loaded, he will probably hire mercs sooner or later, so you need to peace him out before that.
Alternative solutions
If you're lucky, you can use your chancellor to sow discontent, and spark a rebellion. This will only work, if one of his vassals are already discontent.
Assassinate that mother. It'll cost you, and it's far from guaranteed. It might fail, and he will send a counter-assassin after you, which might not be such a bad thing (more on that later).
Also check if you can have the pope excommunicate him. If he has any enemies without a CB, they might use this opportunity to make his life miserable. Likely this isn't an option for you, but it never hurts to check.
Your ruler and your relations
I don't understand why you can't work from your situation as a child – you should have the same options as a normal ruler, only with severly gimped stats and no children. If you have a cute sister, you should be able to marry her off for an alliance. Invite people to your court and force-marry them, if nescesarry.
If you get the "wrong government type" alert, that means you are ruling something you shouldn't. Go though all of your titles and county screens again. Perhaps you are the de facto ruler of a title, that does not actually exist for some reason? Unless this title gives you a load of levies, get it fixed (by creating it or giving it away).
Do you have a dynastic heir? Perhaps an adult uncle or something? You know, maybe he should be ruling instead of you. Maybe you should fall down a well. Here's some ideas on how to make that happen. This will give you more options as an adult ruler, and might even simply end the war inconcluseively due to a change of management!
Conclusion
Lastly, be prepared to accept losses. If you can have him agree to any peace terms that doesn't involve you losing the game, consider it strongly. You will have CB on any lost territory, even if you become a vassal. It sucks, but it's not the end of the world, it's just a new development to your game.
