I am sorry to hear about this concern.
My worry. Is if he is truly constipated or if he is suffering from the colitis again. Certain treatments for actual constipation would actually exacerbate colitis and diarrhea if we guess wrong.
So, considering the problem is persisting, I would recommend taking him into the vet first. My gut tells me anything else at home will only exacerbate the problem and likely abdominal discomfort.