Dear Jenni,
Without seeing your son, I cannot tell for sure. However, a 1.5 mm ventricular septal defect (VSD) is typically quite small and should cause no problems. I do not know over what time he achieved his weight gain, so I cannot say whether his gain has been normal, or not. If he does not get out of breath or get sweaty when he is eating, his VSD is most likely actually small and not interfering with any growth. I would expect that it would spontaneously close by age 12 months, although we do see children in which these defects remain open into adulthood without complications.
Just a PS. at the weight gain you're mentioning he's actually stayed in his weight percentile (22%), so that's good.
So your son is now 3 months and has gained 2.4 kg, correct? It's not a huge gain, but sounds okay to me. Have you seen his normal doctor? As far as I know a 1.5 mm VSD is quite small, and does have a good chance of closing on it's own.
My daughter was born with a moderate VSD and small ASD, however she was in heart failure by 2 weeks, and she will need OHS to close her holes. She has only gained 1.2 kg in 2 months (currently 3.8 kg), which actually isn't too bad for being in heart failure.