Can you do the syringe feeding like... By putting him on a paper towel and then putting one hand behind him to keep him from backing up? That seemed the least stressful way when I had to force-feed mine awhile back.

Can you feel any sort of lump or hard space on the frog's right side? Just wondering. That'll give you an idea of whether or not he's got poo in his system. Just be very gentle when you're feeling for it, since they don't have a lot of protection for their internal organs.