Leather?

Leather can be porous, so I would recommend a commercial grade vinyl. Naugahyde is one brand.
But more important is to purchase a sofa that is "ready to assemble" and that also assembles together using standard module parts/components. Easy to replace any of the components that have been damaged. For instance you can replace or repair a seat cushion or arm or seat deck etc. Even with the same original material.
No more throwing out the whole sofa just because one place needs to repaired.

Here is a link that might be useful: replace or repair modular built sofas

I have a fabric couch; the cushions that we sit on are covered with a twin-sized waterproof mattress pad, tucked around the cushions and secured with those little straps that are sold to hold dust ruffles in place, and then there's a machine-washable slipcover over the whole thing, including the back cushions. I expected that it would be obvious that one was sitting on a waterproof mattress pad, but it's not "crunchy" or uncomfortable at all. If I were concerned about urine on the back cushions, it would be pretty easy to put another twin-sized waterproof mattress cover around those: the slipcover fits over it just fine.

Not the most elegant solution, but it got us through a "pee-on-the-couch" phase without any damage to the underlying cushions. Now if only I were confident that the "vomit-on-the-couch" stage were over... but that one seems to reappear a few times a year!

Whatever you decide to buy, you might want to consider using a twin size waterproof mattress pad on the seating area when your son is sitting on the sofa. It can be removed for washing as well as when having visitors.