Force-feeding should be a very very very last thing to do!
Don't forget it's the time of the year for BP to go off feed. So don't worry. If he isn't loosing weight seriously don't start on force-feeding!!! Force-feeding can only make things worse so never do that till it's really necessary.

What's the temperature and humidity in his cage? Day and night? Have you recently had a stool-sample checked by a vet?