“My 8 year old son's issue?”

My son is 8 I have always dealt with this problem with him. He does not go to the bathroom "every day" every other day are even sometimes every 3 days on and off. It's very confusing. I have spoken about this with his doctor they advised me to switch his diet which I am really putting in that effort. No more junk food more water trying to get him to eat veggies but like I said he's 8. He gets upset when he can't have his fav snacks but he knows it's a healthier choice this diet he is on but this isn't my concern. I got the diet down packed my concern is when he poo's first time I saw this happen in his stool I can't explain this it was very wide and flattened? The texture was like soft served ice cream but in other shapes (lol) one was long but skinny? I'm not sure if this is normal are I am just paranoid. I am introducing "fiber" brownies he likes it just should a child's stool be that wide? As wide if you made a C with pointer finger and thumb he is around 78 lbs and 4'3-4'4 (boys run big in my family).