Hi, my daughter is almost 2 months old, we've been dealing with the same thing. CFers do seem to get worse diaper rashes, just due to all the poop, and diahrrea will really cause some problems.
It seems like he's having very frequent stools, my daughter goes maybe 5 or 6 times a day, maybe 8 or 9 before we increased her enzymes. So see about starting enzymes or upping them if needed.
My ped suggested hydrocortisone. When it gets really bad I apply hydrocortisone, clotrimazole (for yeast), and desitin. Desitin has the highest % of zinc oxide, like 40% or something crazy, where as other creams have around 12%. Once they get sores, they can develop secondary bacterial or fungal infections, so thats why I use the clotrimazole. And hydrocortisone sparingly because it can thin the skin. I use all 3 at once, if there's no rash I use just the desitin.
Let as much air to it as possible. Pat dry after using a wet wipe before applying cream. After the bath when they're all bundled in a warm towel, leave the diaper off for a while.
I'ce been doing this and haven't had a rash in weeks. If it gets too bad, call the ped, it could be a secondary bacterial infection in any open sores, and may need a topical antibiotic.
Good luck!