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.

Eating is a sensory driven issue for many of our ASD kids. Some kids have oral aversions based on food textures. You might want to look into finding a feeding therapist and possibly a Dietitian/RD. Feeding therapists can be either an SLP or an OT. You will want to look for someone with a background & experience with feeding therapy. Most therapists start off with just putting a food on a plate in front of the child, next step is the child just touches the food, then gradually touch a food to his lips & so on. Offer him homemade fruit smoothies to help add some fiber (this can ease his constipation) into his diet, strawberries contain a good amount of fiber & mix well with most other fruits. Also try different yogurts, the Greek style yogurt contains extra protein & fat in it to provide what’s missing in his diet. Be patient & just let him see what you are eating at the table, gradually his curiosity & hunger will motivate him to try what is appealing. Hope this helps, I know it’s upsetting to see your child not eating because we went through a period of time where my son gagged on most foods & it was a real struggle for him.

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