If by underachievement you mean not getting good grades, that can be strongly influenced by the curriculum being so easy for him that it is not worth (in his mind) the effort, voila, bad grades. In our DYS child's case, fixing the environment (end-of-year testing to see what he knew led to a two-grade SSA) led to As in that subject and far better classroom behavior.

However, if he's not understanding the material, that's a different situation and might indeed warrant further assessment.