Yeah he's lost 20g of weight since last month since he couldn't open his mouth he barely ate. Though I've seen him drink plenty in a day. I have started giving him mashed food now and strained water to drink.This is a hard to heal place because he has to open his mouth to eat, and every time he opens the mouth he breaks open the wound. Keeping it soft helps. And to keep it soft, you apply first aid ointment daily. I use Neosporin.
Until the wound is healed and no longer has a scab, you'll have to chop up his food into tiny pieces so he doesn't need to open his mouth very wide to eat.
It's hard for me to tell just by looking at the pictures, but it also looks like he may either be dehydrated or very thin, judging by the amount of meat on his front legs. Do you soak him daily in warm water for about 15 or 20 minutes?