Sounds like your kid is a totally typical kid.  My son (swcah) has days when he will eat absolutely nothing, but he has days when we can’t fill him up.  My niece (non-cah), who is now 10, ate nothing at all until she was almost 4.  When she started school at age 5 she only weighed 32 pounds.  Now she eats everything.  Kids tend to read their body signals better than we read our own and only eat when they are truly hungry.  As long as your child’s levels are normal, I say let him eat when he wants and he’ll be fine.