Sushi is 3lbs. She pretty much stopped growing not long after the 6 month mark, maybe a few oz's. Now she is 21 months old, still 3lbs, very active and healthy. But i have to admit she is super fragile and I am constantly worried about her all the time.
The only major health issue was the extra teeth, which seems to be very common among little dogs. My maltese had the same problem too.
As far as fixing him goes, please make sure the vet that does it is familiar with operating on teacups. It's VERY impostant what they use to put them under, and what kinds of tubes they use to prevent collapsed trachea.
Good luck with the health issues. Will keep your little one in my thoughts
Sending belly rubs to him.