I'd be a little careful with the cabbage, it can add to the gas problem. Do you have canned pumpkin available to you? It's high in fiber and they seem to take it pretty easily. I usually mix the cooked pumpkin with a little bit of Critical care and a pinch of probiotics and syringe feed it to them.
I just nursed a 9 week old Flemish Doe through it, out of a litter of 9, only one got it, and she is recovering slowly. It took 3 weeks, but she is eating on her own and doing well.
edited to add: If he won't take a tummy rub, try just holding him on your lap, on a warm towel or heating pad. The heat helps too.