Hard poop?

Okay so my son is 1 1/2, he started drinking milk right after his first birthday and ever since has pooped little round hard balls, it doesn't hurt him but sometimes he strains a little and I was just wondering is this normal? The reason why I haven't told his doctor is because I thought ya know maybe it was the milk and his body was just adjusting to having it instead of formula but he still poops like this.😑