Personally .... I'd talk him out of it. Nothing special .... doing that 30 years ago maybe special ... these days it's almost like proposing on a bus. Just my opinion.
I'd rather propose at the destination.
P.S. If he still wants to do it on the plane ... he should do it towards the end of the flight AND .... a lot of effort from the crew to make it special .... like using the PA system, etc. Otherwise .... change the venue.