First, apologies if I'm forgetting backstory you've told before (I had a quick look but didn't find any) but what is known about the cause and prognosis of your DS's hearing loss? Is it expected that it'll be permanent, or is it treatable or something that may improve?
At about that age (for roughly 18 months, when I was 4-6, I think) I was deaf [and gifted ;-) ] - apparently I was "profoundly deaf" for at least some of that time. Irritatingly I know practically nothing of the medical details; my parents didn't keep notes or remember much and the records have since been lost. I had a long history of ear infections and this was probably just extreme "glue ear". What I remember is going to the hospital, seemingly often, to do hearing tests, which was quite exciting, and having weird daily exercises to do such as blowing up balloons with my nose - something about needing to open my Eustachian tubes. I recovered completely.
I got very good at lip-reading - it may be that this led to the problem not being spotted before it got really bad. Presumably giftedness helps at least a little. It might be worth making sure that people who interact with your DS are aware of the contribution lip-reading can make to their being understood - tell them stuff like keeping their hands away from their mouths, making sure the light is good and DS is looking at them before they speak, etc. That's easy for people to do and can really help, in addition to whatever else.
If your DS's hearing loss is likely to be permanent, totally agree with MegMeg that it would be good for him to learn a (real) sign language starting ASAP. Indeed as with any additional language, learning sign language would be good for him anyway.
