If you have the numerical scores (not just the percentiles), they are not the same as IQ, but are supposed to be on the same "scale" as IQ (100 average, SD 15).
For purposes of answering the question from the specialist, I would say that you don't know his IQ because it has never been tested, but his CogAT scores are ____. That way you have given them all the information you have, and they should know how to interpret the CogAT information better than you or we do.
Have you tried giving him graph paper to do his math work on? That was helpful for my DD until she (mostly) mastered keeping everything in line. If he is able to show work when given graph paper to write on, that is definitely an accommodation that you should ask for that should be very easy for the school to grant.
