It is probably normal if he is eating well in spite of sleeping a lot. Did he have a PDA? Mine were 2.5 months early and never slept so well, at that age they were taking 2-3 naps per day and sleeping about 3 stretches of 2-3 hours at night(feeding in between). Now, at 6 months or 3.5 months adjusted one baby sleeps 8 to 10 hours most nights, while I am lucky to get one 3-4 hour block with the other one.
So, if he is eating well and developing well, you might just be lucky to have a good sleeper!I would keep a close eye on his weight if he has a history of PDA or other heart issue.
So, if he is eating well and developing well, you might just be lucky to have a good sleeper!