My 3 month old baby maybe constipated what can i give him?

my baby hasnt done a poo in about 3 or 4 days which is unusual and his last poo wasnt very big like his normal ones, hes farting alot and crying more then usual, i tried giving him his bottle and his toys nothing would work, what can i give him to help him go to the loo? i tried water and he wouldnt take it. what is he allowed for fast relief?