Tinnitus coupled with PTSD could do what he's experiencing. I have tinnitus myself and if I'm not hearing something I hear a small "reeeee" type of noise but once every week or two weeks I'll get a massive one that drowns out all noise for a couple of seconds and it takes like 15~ - 30~ seconds for my hearing to go back to normal. If he associates the accident with the ringing then he'll flip out whenever he gets that episode. My normal hearing isn't terrible but it seems like his got really damaged on one side so his episodes may be more common.