Depending on how much you want to reduce recoil energy, you might find that the load shown on this page will help: http://www.hodgdon.com/PDF/Youth Loads.pdf
A stock that fits, with a good recoil pad (Kickeez, Limbsaver, Pachmayr) will reduce felt
recoil appreciably. In my experience with quite a few new and young shooters, muzzle-blast often effects how they interpret recoil as much as the kick, itself, so don't neglect to use very good hearing protection. If they aren't used to shooting guns larger than a .22LR, I would recommend ear muffs over top of ear plugs. Yep, use both, for new shooters.
The factory .243 Win ammo doesn't kick all that much to begin with. If the Hodgdon Youth Loads don't appeal to you, I would consider using the 85gr Nosler Partition bullet over the most accurate middle to low-end charge, for velocity in the 2800-2900fps range. With a good stock, recoil pad, and some practice, I bet your young shooter will be pretty comfortable shooting that 243, in just 2 or 3 range sessions.