My 5 y/o has newly developed an asymptomatic black spot in anterior frenulum of his tougue. he had a skin lesion excision which turns out to be dysplastic nevi. should i get him checked for it?

Absolutely: Particularly since he has already had a dysplastic nevus, he may be at greater risk to develop melanomas. Any black or multicolored spot on his skin or a mucous membrane (like in the mouth) should at least be checked out by a dermatologist (or, in this case, ear, nose and throat specialist).

Mucous membrane: Yes. You should have him checked out by a physician to determine what this may be. The history of the dysplastic nevi should lower your resistance to having this examined.