In terms of the sight-singing in the aural, I would just drop a note to the ABRSM Special Needs co-ordinator suggesting that this boy might not be willing to undertake that particular test. OK, so he's not going to get 18 marks if he omits a bit but, if the rest of the tests are good, he's probably not going to fail the aural either.
However, you say he doesn't read music? How's he going to do the sight-reading on the piano? The only dispensation he'll get is 2-3 minutes to look at it, rather than 30 seconds. I am a bit confused by your comments here as to quite what you mean.
I agree with Noodle that TG might be a different option for him.
If in doubt, contact the Special Needs office at ABRSM for clarification.
Hope his exam goes well if he takes it.