Do you mean a true stroke, or an IVH? I've taken care of many, many preemies with IVH of different degrees (I-IV). Their outcomes vary widely depending on how bad the bleed was, how well it resolved on its own, and what the rest of their NICU course was like.
As far as true strokes, I've only seen a few babies with these, and none of them happened while they were on the NICU. Nuclear medicine studies suggested that all of them happened while the baby was still in utero. All of these babies had poor outcomes developmentally, nothing like your son who is running around.
Either way, I'm glad your son is doing well. If he has some learning disabilities later on, you can deal with that when the time comes. My sister-in-law is a first grade teacher and she sees a lot of ex-preemies who need some extra help with school and get different therapies. So he won't be alone, and they are getting better and better at therapies for these disabilities. GOOD LUCK!!!