M.P.
I suggest that flat affect is an issue unrelated to ADD. It can be a part of mental illness or autism. Or it can be a result of feeling badly about ones self. Or it may be as simple as being shy or uncertain about how to act.
Both of my grandchildren have been diagnosed with ADHD and my grandson is somewhere on the autism spectrum disorder continuum and both of them are very animated.
Does he do well in school except for not participating in class? He has friends and participates in sports. He sounds OK to me. Have the teachers said why they are concerned? Perhaps it's just that they want him to participate more in class. Participation is often a part of the grade. It is important in letting the teacher know what the student knows and how they think.
I suggest that you talk with him about class participation. Ignore the flat affect comment and ask him if he knows why he doesn't participate. Does he lack confidence? Does it not seem important to him? Is he bored? Talk with the teachers and the school counselor about ways to help him participate.
If the teachers are suggesting that he has a learning disability then ask for an evaluation thru the school district.
Another thought. If he doesn't participate because he's unable to focus on what is happening that is a part of ADD. But usually students who do this are animated. They're just not paying attention and are in their own world. Are they describing that as lack of affect?