First of all, it's awesome that he's only HOH but you're open to him learning ASL! That rocks! Is he in school? You can send him to a deaf school or dhh program, so he can learn in a dhh friendly setting. HOH kids can and do benifit from ASL. Insist that you want him to learn in a deaf program, with ASL. ASL and Deaf ed is NOT just for deaf kids!
Your audi is WRONG. Even unilateral loss can sometimes come with a spoken language delay. I have a syndrome, where even the hearing kids with it often have severe spoken language issues. You know......I wonder if the problem is that you might need to see a specialist audi....like one at a teaching hospital. ABRs can be inaccuarate. My ABR says I'm deaf, but I'm actually HOH....and I know other people who have experianced disreprencies on the ABR. .......I know you're concerned if there's anything that you can do re: your son being HOH, but did you know that back in the old days a lot of HOH kids, and even deaf kids didn't get HAs/CIs until we were toddlers! We still learned to speak and hear relatively well.
What state are you in? Have you contacted the state's Deaf School?