I have been giving him specific instructions. Like read the instructions twice.
I'm no expert and I'm only speaking from my own experience here, but I'm not convinced that this is specific enough. I used to tell my DS9, "Rework the problem to check your work," but I found that he just made the same mistakes the second time around. I think that the ability to check your work effectively is a skill that has be learned, rather than something you can just do.
DS9's improvements came when I got really, really specific: "The LAST thing you need to do is check signs."
In your son's case, you might say, "The LAST thing to do is to check that you crossed all your Ts and dotted your Is," or "The FIRST thing you need to do is check if the problem is addition or subtraction."
I think part of the problem it's not such a big deal because he is missing only a point or two.
Hmm. If it's really only a point or two, is it even worth talking about? Again, this is just my opinion/experience, but I'm cautious about not encouraging perfectionism in cases like this with my kids. If you mean it's a point or two on the Ts and a point or two on the Is and...and...and..., then it all adds up to a big loss of points, that's different.
Val