Newborn diagnosed with VSD

Hi,

My son is 3 weeks old now and was diagnosed with a medium size (1.27cm) VSD when he was a week old. For the past two weeks he hasn't been growing well and only put on 7grams a day. The paed said he should be putting on at least 20 grams a day. My questions are as follows:

1) His cardio paed said to monitor his growth for next 4 weeks, and if he is still not putting on enough weight, he may need to undergo a surgery to close the hole. Is this necessary? Are they any other form of treatment? And if surgery is recommended, when is the right time to do it, now or when is turns 12 months?
2) He went to see another cardio paed for a 2nd opinion and she prescribed him two medication (furosemide oral solution 10 mg/ml and potassium chloride) because she says his lungs is swollen. His original cardio paed has advised however that if he is breastfeeding alright, there is no need to take the medication. He has left it up to us to decide whether to give him the medication. We're confused as to whether we should start him on it and if there are any side effects to the medication for such a young baby. Pls advise.
3) Should we start him on a high calorie formula to help his growth. He is fully breastfeeding at the moment as I believe breast milk is best for him.

Thank you in advance for your help.