I hope he gets better soon!

I haven't read through all the posts, so someone might have already suggested this:

Get some vitamin C powder and put 1/4 a teaspoon in with a syringe of baby food or mashed fruit/veggies. If you use mashed fruit, water it down. You can also feed him water through the syringe. Feed him every 2-4 hours depending on how he's doing. Just don't give up on him, some times the hand feeding can go on for a couple of weeks or he might only need it for a few days until he regains his strength. Hopefully this will work.