My Gwyn's had the constipating form of IBD for 16 years now, and it's always a constant battle with us

We've pretty much always used prednisone, and have at various times used pumpkin, lactulose, cisapride, cascara, Zantac, Metamucil, a raw food diet, vitamin B-12 injections and probably several other things that aren't occurring to me at the moment.
Like a lot of other constipation kitties, we're currently using Miralax. Start with 1/8tsp twice a day, and *slowly* increase as necessary. You'll definitely need to add extra water to the food as well to let Miralax work properly.
How old is Johnny? A lot of older cats have lowered kidney function (the average age for diagnosis on that is 9; half the cats are diagnosed before age 9, half after). Lowered kidney function cat trigger various degrees of dehydration, and any amount of dehydration can contribute to constipation. So any older cat with constipation issues, I urge them to get their kidney panels done and to do a consult with their vet on the costs/benefits of starting sub-Q fluids earlier than normal.
Regarding raw food diets: cats on a raw food diet tend to defecate less frequently than cats on a more 'normal' diet. A raw food diet is fine for most normal cats and can be very beneficial for cats with chronic diarrhea problems and some cats with constipation issues. However, in at least some cats with constipation problems, a raw food diet is *very* contra-indicated. The handful of weeks we transitioned onto a raw food diet saw the worst progression in Gwyn's intestinal problems than any other period in the 16 years we've dealt with constipation. So if your cat is prone to constipation, please monitor her bowel movements carefully if you switch to a raw food diet. It can help some constipating cats, but it honestly was the worst thing we've ever done to Gwyn.