I am really sorry to hear this.
It sounds like your guinea pig really needs to see a vet. It doesn’t need to be an exotic vet, just any vet will do.
This sounds much more serious than overgrown teeth, wether they are digging into his mouth and causing so much pain he’s acquired a head tilt or wether there is something else going on.
I am sorry to be so rash but it does sound like your piggy is in a lot of pain and there is a chance he will have a slow painful death if you can not get him seen, please keep his as comfortable as possible.
You can offer him blankets to lie on/with, keep the room warm (not hot) and begin support feeding asap. Please try and get some water in him, you can hold him and then bring his water bottle to his mouth and hopefully he will try to drink from it. If not you can very gently squeeze the bottle and hopefully he will swallow the water that is realised, do not force the bottle down his throat or squeeze it hard so he gets caught off guard. Again I say please do this very gently as you do not want to cause him to aspirate.
When you phone the vet, make this very clear it’s an emergency. I understand that vets are sometimes far and few in America but this may be life or death.
I can tell how much your love your piggy. In reality there is only so much you can do as pet owners and as they are so good at hiding their pain, sometimes we don’t realise/find out about things until it’s too late. I can tell how much you love him and that you’re a great owner so his chance of recovery is now going to come down to the help you provide him now, but I really wouldn’t worry yourself saying you should have caught it sooner.
My thoughts are with you in this time xx
P.S - I am going to attach the forums feeding/diet guide, as cooked vegetables should never be fed, so this may be helpful for future use.
Long Term Balanced General And Special Needs Guinea Pig Diets