Can't have your cake and eat it too.
If his flight/anti-gravity comes from solar, and as a child those powers were only beginning, then a child with a dense molecular structure would have been very, very heavy and the Kents wouldn't have been able to lift him, and he'd have broken floors, chairs, beds, desks at school, etc. ("Birthright" said Krypton was 30 times as massive as Earth. Multiply a normal child's weight 30 times.)
I like the idea of him still being an analog to Kal-L if he loses his solar powers, but it creates too many problems, and also it wipes out the chance at stories where he does lose all his powers. (I mean, you can't keep putting him under red suns on Krypton-sized worlds just to do such stories.)
Actually, could have it established that his mind can control body density to a degree, so would be subconsciously altering it to remain like a normal weight due to damping his density factors, that is where the yellow sun energies kick in!