Are you hand feeding? It's common for them to be in a lot of pain after a major surgery like that so most need to be hand fed. If you are hand feeding, most you mix with water so it goes though the syringe, and since chins don't drink much he may be getting all the water he needs in his food.
I would most definitely not give him lettuce! That will likely cause him to get diarrhea, which would make any dehydration much worse. Chinchillas are not meant to eat vegetables, or fruits, nuts, seeds, or animal products. If he is eating enough on his own (a couple tablespoons of pellets and some hay) and you aren't hand feeding, you can try syringe feeding him water. Be very careful not to squirt it his mouth, just drip it and let him lick it up.