CAMom. I would not make too much of the subtraction facts. Dh is dyslexic (and very gifted) and he also has a super hard time with subtraction. But honestly, being a math teacher, I would say that it is very common for many students to be much better and faster at addition/multiplication than subtraction.
I think it is important to keep a close tab on him and see whether he can keep up. If, after helping him out, he still is not catching up to his peers in just that area, then it might be time to start searching for specialists.