Need Suggestion about the Open Heart Surgery of my New Born

[Bilaljameel19]

My 7 weeks old son has a large un-restrictive VSD, He has also trisomy 21. In his initial echo reports he had a small PDA, small ASD and Large VSD, now ASD and PDA have been healed.  In his recent echo, doctor remarks that quoted "LV Volume Overload" and "Severe PH". His weight is now 3.5 KG
We consulted with Paeds Cardiac Surgeon and he is ready to do the surgery. But his cardiologist was saying to wait till his weight reaches at 4.0 KG. I need a suggestion that will it be okay to operate the baby now with severe PH? or should i wait to turn his weight to 4 KG. He is taking feed through NG and we are using special pre mature formula milk. i have heard that there is a high risk of mortlality with severe PH. Is that right? need suggestion.