Hi there. Ya know, I have never been a huge breakfast person. I know it is an important meal but am often just not hungry in the am. I force myself but as a child, my mother enticed me to eat with foods that were not traditional breakfast foods. A peanut butter and jelly sandwich was sometimes my breakfast. I also did like oatmeal. A hard cooked egg. But sometimes bean soup. My mother tried to make sure I got some protein in the morning.
My son is constipated as well. I give him a cup of grape juice in the evening, it helps tremendously. While breakfast is great to have a bit of yogurt with some fruit and some fruit juice, it isn't his only opportunity.
Agree with anniebrooke that a pediatrician could be helpful for the problems you mention. Peace and good l uck
Talk to his pediatrician. Try oatmeal in the morning instead of juice or fruit. Get him checked for encopresis. Good luck!