@ttobler If it is IBS, the reason he is not eating despite his hunger is the irritation in the stomach. Basenjis eat grass and throw it up, usually with just a bit of bile, in an attempt to relieve discomfort.
Try him on a very bland diet of boiled rice and fish as I have suggested elsewhere recently. A coddled egg on a little bit of rice usually goes down well -
and I would get some form of electrolytes to prevent dehydration. If necessary, a little honey melted in warm water for him to drink as an addition to the normal cold water bowl always available (I hope !)
I have a bottle of concentrate electrolytes always on hand so I can offer it at need. Dogs love it !
If he eats when he is REALLY hungry, like after his enforced fast, then it has to be something which is making food either taste nasty or cause discomfort.
Try not to keep him on steroids for very long but obviously, be guided by your vet.