The VSD is with 3mm rather small. It can still close on its own.
The pulmonary valve stenosis is the major problem.
It is often solved by surgery or balloon dilation.
Of course balloon dilation is much less invasive but I do not know if that is an option for your son. You can discuss this with his doctor.
You do not mention other health issues, so it is likely that his weight does not increase because of his heart condition. Once the valve problem is solved, he should develop normally.
If the repair is successful the outlook is normally very good.