Well, unless they are planning to sell him they seem happy with him.
Approach them slowly, maybe contact them through email or over the phone, tell them that he was your first horse and you would love to see how he was doing, ask if you could visit him. If they don't seem like they're interested in selling him I wouldn't bring up the idea, but instead as if you can come visit him every now and then, care for him a bit, maybe ride him? :] I'm sure that would be a compromise they would be really pleased with if they weren't looking to sell him.