Constipation

Hi, My 17 month old boy cries really badly when he is trying to poop. It usually takes quite awhile for him to release everything. There is even a little blood in his diaper most likely as he had to push out a hard and large stool. He eats fruits every alternate days and he drinks plain water too. He eats normally and is still on formula milk. I am so stressed everytime he is about to poop because he will really cry the house down. He goes back to his cheerful self after he is done pooping. Can any of you suggest ways I can help him get out of this phase please?