I think it would be helpful to know some additional information, such as...
- Has your son flown before?
- If yes, what has been his longest flight?
- If yes, has he ever had to make a connection/switch flights? How did he do with that transition?
- If he's never made a connection, what is your plan if he doesn't want to get back on a plane?
- What leads him to become restless? Boredom? Being confined to a seat for an extended time? Tiredness?
- How do you handle it now when he becomes restless?
- What are his behaviors when he becomes restless and are they such that it could cause problems with other passengers/flight crew?
As for which airline would be "most understanding", it's not a matter of understanding so much as what they are allowed to permit with regard to behavior before it is seen as a potential risk to the flight or to passengers/flight crew. An individual flight attendant could be very understanding on a personal level, but if he becomes very loud and disruptive, it could very well become a problem.
If he's never flown before, I wouldn't even consider this trip, but that's just my personal opinion. Flying to Hawaii from just about anywhere other than the West Coast is a pretty long and tiring flight.
Best of luck to you whatever you decide!