I've been killing deer (50+) with a .243 using Searra 85 gr. HPBT Gamekings for over 30 years. I also have a .243 WSSM (BOO) bad inventions all WSSM's in my opinion. Barrel burners so much that the manufactures have to chrome line the barrels to get 1,500 rounds through them. The only ones that shoot 5/8 inch groups are maximum loads. Same with the .25 WSSMs I have experimented with. The Winchester .243s (not WSSM) are more accurate also.
The WSSMs have a noticeable sharper recoil than the older .243 even though the numbers may be similar. Boo for WSSM and Yea for .243.
If a .223 is legal for deer where you live why not load .55 grain bullets in the .243 and get your young one to pop some crows or woodchucks at 300 plus yards. They will learn how to shoot more accurate if he is not scarred of the recoil AND be able to hit them consistently at over 300 yds.
When he gets more meat on his bones load the .243 up to the .85 gr HPBTs and he will be set for life with an outstanding shooting rifle for deer and varmints.
Stay away from the WSSMs.
Just my .02,