Sounds awesome to me! Your son is sleeping, gaining weight (doesn't matter how much) & has plenty of wet/soiled nappies - all good indications
Some babies gain fast, others slow. My DD was a slow gainer & still is at 4 yo. Her 3 yo cousin weighs more than her, is taller than her & has bigger feet! And there is not much height difference between our families - we are not tall people!