well my tips are let him eat what he wants when he wants. i know this sounds crazy. but its almost like they'll go threw cravings wanting a certain food then not intrested so have one containe or bag of fav. foods always. Also sneak! be sneaky!
add very small chunks of fresh fruits or fresh veggies to muffins. prebitoics to drinks, and vitamins liquid or gummies..
For constipation try hand squeeze orange juice, apple juice, grape juice ect. offer water as well.
Just some of my tips.

Agree with hopie. Let him eat what he wants, you'll drive yourself mad trying to get him to eat "healthy" all the time. Our son always eats sand so he gets constipated. The pediatrician told us to give him miralax and it definitely helps. It also helps that our son loves a lot of fruit