The "dial tone" sounds like a variation of tinittus which is simply ringing in the ears. Sometimes it's like static or "pink" noise.

He should take a decongestant or antihistimine prior to flight. That often helps. I am not sure that flying would exacerbate the damage in any case - but the decongestants might add a little comfort level. Drinking a lot of water in flight is a good idea also - there is very low humidity in aircraft.