Stool Withholding in 3yo since potty training

My son just turned 3 this month and was doing just great at recently being potty trained. He would go to the potty for urine every time and never had accidents. He would even go all night without a diaper and wake up dry. But since the beginning has had a bit of difficulty with bm. The sensation of releasing it into the toilet almost seemed like it was scaring him. He would suddenly get all panicked and say he didnt like it. So I didnt want to force him and he would ask for his diaper to poop. I didnt want to make an issue of it so I would give him his diaper but now he still says he's scared and runs around and holds it in. He just held it for 5 days and I gave him a glycerine suppository and he finally went. Im so worried and dont know how to help him. Please advise.