is my son under weight ?

i feel like my son is under weight compared to other babies & my mil keeps saying he's to small which irritates me a lot bcuz she compares him to her other grandson whos 8 months & 28-30 pounds, my baby is 5 months 16lb he eats well always playing & very happy so i don't feel i need to be concerned about his weight ☹️ do i?

-we are both pretty tall he's 6'2 215lb i'm 5'9 170lb , i did have something called Low Pap A and the doctor did advise me my son would come out alittle small he i was 40 weeks on the dot & he weighed 6lbs.