I'm just a newbie, but have you tried feeding him through a syringe? He might not like it much, but if it gets some food into him, it's got to be worth it.

I found these sites which may offer some useful tips:

http://www-personal.ksu.edu/~sprite/feeding.htm
Hide-E-Hole Ferret Rescue Inc.*-articles-Force-Feeding Duck Soup
What should I do if my ferret isn

Good luck, we all want to hear about this little chappy's recovery.