It certainly could be the LD, but my son also plays chess and I see a lot of inconsistency in his performance. It's true of many children in his age range (my son is 7--IDK how old yours is), perhaps most especially those with a lot of potential who do not practice that much. That is, they may have flashes of brilliance but also they may blunder and miss the obvious.
There is a lot of grunt work in chess...much more than I first realized. Your son might benefit from playing and replaying positions against a computer until they're really solidified in his brain. Chesskid.com has some great exercises, but you have to pay for the really helpeful stuff.
