If you're concerned...call your doctor and insist they check him again. Don't worry about being a paranoid mother. It's better safe than sorry. At this point since you discovered something was amiss the last time....I'd be having them double, triple, quadruple check everytime I had a small doubt about his spit up. Hopefully it is normal and it's just his intestines are still on the mend. But I am sure no one hear can guarantee it.
I will say though that it is normal for babies to spit up...even some a lot. It does sound like he's doing well with gaining weight and his bowel movements and everything. so that's great. But if ever in doubt----I'd make them check it back out.
Good luck!!! Congratulations on your new baby. I hope he is healing well and that this is the last of the surgeries.