Wow. That's great wording ASDmom.
I have to say, as a former special educator, I wouldn't worry about offending them at all. This is SO normal. And we rely on parents to help us hear the kids out. They so often don't want to say anything at school because that will just embarrass them more. On the other hand, we have the responsibility to help these kids in the classroom, so we want to make sure that autistic kids, who don't pick up subtle communication, know that they have help. But if he's getting the help, and is feeling self-conscious about it, they need to know that.
Please, don't feel weird about saying this kind of thing. We hear it all the time. These are teen-agers! And if your son's helper is at all experienced with this age group, she/he won't be surprised or hurt by it at all.