for what it's worth, here's the recent history of AA266 from Lihue to LAX. You can see that most days it arrives earlier than 12:48am (looks like the Sat flight is a later schedule). History won't do him any good if his particular flight is late, but odds are more in his favor. I would agree with your premise that T4 in/out is ok with a 40min layover (both flights on AA), but don't have any experience with T4 to T5 transfer.

According to the LAX Airport website it should take your son 8 minutes to go between Terminals 4 and 5 using the underground tunnel. Go here https://www.flylax.com for more info - use the "Find My Way in LAX" link and enter starting and ending terminals in the search box - you'll be directed to a map showing distance, time and route.

Whether the scheduled 40 minutes between flights will be sufficient time to make the connection will depend on if your son's inbound flight arrives on or very near to the schedule, where your son is sitting on the plane and how long it will take him to deplane. 40 minutes should be doable but he doesn't have any wiggle room to dilly dally.

The risk is he'd have to wait to take the next available flight, which would be ~6am at the earliest if there is space. There are plenty of AA non-stop flights LAX to DFW.