If he is not eating hay then you must step in and syringe feed him a recovery feed or mushed pellets. Hay is 80% of the diet and I’d he cannot eat it independently to maintain his weight and gut function then he will quickly become unwell. Lettuce is not enough to sustain them.
Ensure you switch to daily weight checks so you can ensure you are getting enough syringe feed into him in each 24 hour period to keep his weight stable.
Teeth do regrow but you need to see a vet to ensure the teeth are not too badly damaged.
You should also see a vet to check he is otherwise ok from internal injuries. Any fall from height requires piggy to be checked by a vet.
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures