The problem if you try to force feed him is that you can cause him to aspirate so it's never a good idea.
Instead, I would give the vet a call and get a prescription diet that dogs seem to love. It's packed with calories and will help a lot.
You can TRY a little boiled chicken, cut up in small pieces AFTER putting a little karo syrup in the mouth to raise his blood sugar. Jarred baby food meats also work well.
Let me know if you want to discuss this more.