What disability was he diagnosed with, out of curiosity? Was it dyscalculia?
RightStart might not be a bad choice because it supposedly has spiral aspects to it (i.e. the curriculum periodically "spirals" back to reinforce topics already learned). I guess my opinion would have to be based on more knowledge of whether he does well with conceptual learning but has more trouble with math facts, or something else is going on.
I personally have a very high opinion of Singapore
Math for elementary grades, and think that if it were correctly administered that it would work for almost all children very well. Some advantages for your situation include a progression from concrete (hands-on work with manipulatives, especially in earlier grades) to pictorial to abstract; the ability to add extra work at each stage, through workbooks and extended practice sets ("intensive practice" and "challenging word problems" books); strategies for mental math and problem solving that work well (though one could fairly claim that the over-application of the SM bar model can be confusing); clear guidance for the instructor through the teacher's manuals and similar home instructor's guides; and in general a very thorough and clear presentation of topics. It would also be easy to add extra practice with an online or written program at each step, though I suppose that's true with most programs.
There are some thoughts and links relevant to math facts practice in another thread