My son doesn't have trouble at all when his teeth comes out (all four from below, weird!), and when I brush his teeth, he really enjoys it and he even take it from me and start bitting it, so I let him bit the brush, (not the one with bristles, just soft plastic with some soft edges sticking out). See if that would help. I guess you can do much with his appetite, maybe give him some juice, since it's sweeter, give him whatever he likes to keep him taking more food in even it is not milk. I make pancake for my baby that helps, since that kind of solid food you can add lots of different things in for nutrients, and it is dryer so he will eat more without realizing how much is taken.
See if it helps
Is he just allergic to milk? If so, it doesn't mean he will be allergic to other things, it's different, because in his body, he is lack of (I can't remember the name of that thing, but whatever...) That doesn't mean his immune system has problem, so it's ok to eat other things. Well, but some said Fish is somewhat easy to bring out allergy, so yes becareful if you are afraid, but other things should be fine.