I drive a 2005 TJ, and that in-line 6 generally runs forever and takes all kinds of abuse on trails, especially in a Rubicon. There's lots of used, low mile engines out there, or you can go the crate engine route. Have your son find the local 4X4 shop that the off-road guys use, and they will have contacts with the right suppliers. Here's one, but I am not giving them any recommendation, just a quick google connection and I generally stay away from this type of "SuperStore", instead looking for the small, independent shop that the ruff & ready off-roaders use.
4 Wheel Parts Performance Center Truck and Jeep Parts - Installation stores-RH3
Here's another link to engines, from long blocks to crate. Again, no recommendation, but there's tons of Jeep parts suppliers out there, supporting a huge on and off road group of Jeep owners.
JEEP ENGINES & MOTORS | Rebuilt Engines
There's also stroker motors available if he's wanting more power, but usually that's done with gearing...
Jeep Strokers : Titan Engines, Jeep Strokers Remanufactured Engines Engine Kits
Start by finding a good little 4x4 shop and things get easier quickly.