I agree with the other moms who advise you stop the solids and concentrate on formula. He won't drink if he's filled up on any food. Actually I'm surprised your doctor would recommend solids already, especially in view of the limited formula intake, which is more important than solids for the first year.
It's good that your son is having wet diapers, growing, and content, it means he's healthy. Make sure your son is getting exercise, on his tummy is good as he'll reach for things and try to move himself and will need more calories. All babies have different appetites, but won't drink much if eating solids, so cut those out, keep your pediatrician in the loop and ask for a 2nd opinion if he doesn't drink more after stopping solids.