My daughter, born at 25 weeks and had a grade 3 bleed (resolved after 4 months). Today she's 13 years old and is doing well. She too has enlarged ventricles (as of her last MRI that was done almost 7 years ago). We were also told it was likely a result of the bleed but that it likely has no impact on her.
Its great that your son is progressing and doing so well today. Try not to worry. If he's meeting milestones that's all you can ask for. Its great that his neuro wants to follow him so it's very likely that if he begins to struggle you will have a doc already on top of things and you can begin therapy services again as needed.
All the best to you and your son!