OP, if you are willing to invest some time in this, please read How to Get Your Kid to Eat, but Not Too Much by Ellyn Satter.
Also, I just wanted to highlight this:
A lot of time at mealtime, he decline the healthy balanced food I want him to eat.
Young children need a lot of fat in their diet, and as a society conditioned to think fat = bad, sometimes we don't recognize our children's need for it.
My son will gobble up the cauliflower with cheese sauce that I cook for him. I also add butter to other veggies like broccoli. Since he needs fat anyway, it is fine to combine a very healthy vegetable with some fat to make it tastier.