You can try diaper rash creams/ointments or Calmoseptine (there's a link at the top of this page). The product I found most soothing was Nupercainal but not all drugstores/pharmacies carry it. For really bad butt burn, Ilex works wonders because nothing gets through. Unfortunately, Ilex is going through a revamping of their container so it's hard to find right now. But do a websearch and you may find a site that still has some in stock. You can find Nupercainal on the net too.
Many here have installed a toilet seat bidet (there's also a link - COCO - at the top of this page). If that's not feasible, you could try a sitz bath or bath/shower immediately after a bowel movement.
Try bulking up his stool with bread, pasta, rice, potatoes, etc. You might also want to do an elimination diet by removing one of his regular foods every 2 days or so to see if something he's eating is causing him problems right now. That doesn't mean that a certain food or liquid will always cause problems. While his j-pouch adjusts some foods may be a little harder on the new plumbing. If you find that is the case, try eliminating for a couple of months and then reintroduce it.
There are many, MANY different things that can cause butt burn/itching: wheat, tomato-based products, fruits, vegetables, eggs, chocolate, sodas and sports drinks. The list is long but it's only long because
some people have found that something causes a problem for
them.
Often people think that not eating will help alleviate the problem but often the opposite is true because that causes bile production and bile burns and itches. So more and smaller meals might help.
Welcome to the site. I hope you find a solution for your little guy's burn and itch.
kathy