I missed this the first time around. I have a Jeep Cherokee with the same I6 4.0 engine in it.
They already have a bit of intake whistle, step-up to a CAI and you will get more whistle. Of course, the real reason for a CAI is better milage/performance. Get him an Airtube from Rusty's
, or build your own using exaust pipe. Just stay away from the K&N garbage filters.
Now, if he does any offroading and you want some water protection - go with this snorkel
. Best CAI out there, keeps you from hydrolocking the engine if you do any offroading where water/watercrossing is involved - and it makes a hell of a whistle.
Swap in the CRD from the Libertys
160hp/300ftlbs and 28mpg all day long!
Dear god no. The 4.0 I6 is one hell of an engine, taken care of they will last 500,000 miles without batting an eye. While a good diesel would be nice, the weak-ass crap they put in the Libbys isn't it.
EDIT: Oh, by the way - there IS a Supercharger that fits/works on the 4.0l I6. But, not sure if you are looking to spend $3,000 on your son's Jeep.