Please check the twist of the barrel FIRST! Some of the old rifles had a 1:12 inch twist & will not stabilise 100gr bullets. Winchester made 1:10 inch & later Remington 243s are 1:9 (old 6mm Rems were 1:12). I have seen European rifles like Voer & Krico with 1:12 twists.

If your rifle is definitely 1:10 or less twist, then great. If not, you are better of taking a 6.5 cal for the young man. He would get great results with a 125 gr Nosler Part at 2900+ fps & mild recoil.

For the 243 with fast twist I would agree on the above post for a 100 gr Nosler Partition or a Barnes TSX.

Happy hunting & good luck to the young man!