Suggest Ways To Potty Train A Child

We are having a very hard time getting my 3 year old son to potty train he absolutely refuses to poop on the toilet....Im starting to think he may be constipated. He will go 5 to 8 times a day in his pull up, it is normal consistency but also there usually isn t very much of it there. Does this sound like constipation? If so what can I give him to help him go.