They only need the teeniest amount of porridge, it's very high carb so you need to get the right balance between enough for weight gain and too much for icky guts. I usually give about a teaspoonful soaked in warm water. If the bun is used to banana you can mash a little of that in with it too and they loooove that
Also now spring has sprung, fresh grass is richer than hay so getting that might also help both his weight and his teeth.