I do some work for a car dealership, with a large parts department. The newest member of the 'goods in' team is deaf, but he's due to start his fork lift training soon.
I can see no reason why your son couldn't be employed as a fork lift driver.
There are some jobs you cannot do if you are deaf obviously but I dont think this one would be a problem. Remember many forklift drivers wear ear defenders.
One think maybe for him to try it to ensure it does not interfere with the aid.
One of my daughters is profoundly deaf and I have always encouraged her to try things, which she has done and gets on well.