Fire your sons setup a few times and get a good feel for it. An electro is a lot different than a rental for sure. go to your local store and get your hands on a few different models in your price range. Have the dealer throw a tank on each so you can shoulder the gun. Each one feels different. Find what's comfortable. Hell, if the dealer really wants to sell a particular gun, they will probably load a few rounds and take you to the range if they have one.
The biggest thing about gear imo is don't be pressured to place a price tag on performance or quality. My Axe setup with loader and custom Crossfire still costs less than base models of the higher priced guns. It shoots like a champ and has afforded me the ability to buy equipment for my son and I without breaking the bank.
Boss powered Empire Vanquish
68/45k Custom Crossfire