I remember being young and hearing all about shotguns kicking like mules and rifles that would literally knock me down. The first rifle I shot was just before my first deer season. It was a 30-30. Needless to say I was scared 💩less the first time I pulled the trigger as to what was going to happen. I remember it as vividly as shooting my first buck and it was 40+ years ago. WHAT A RELEIF! It just thrust my shoulder back and I survived. Never was I afraid to shoot anything after that. I realized how much everyone exaggerates and how much my worries were not rational.
With all 3 of my sons and nephews I did THE OPPOSITE of what everyone did to me. I always downplayed recoil to them and explained how everyone enjoyed the exaggerated arh arh arh talk.
Never had an issue starting them out. The first rifle my second son shot was my 270 with no recoil pad. He couldn't even hold it up and stay on the target, shot off some pillows. As with all of them, I made sure he had a firm enough hold, snug against the shoulder, and face adequately spaced behind the scope. Then immediately after the shot regardless of the look on their face I simply say, see it's not bad, just pushes your shoulder back hard enough you know you have to respect and be ready for it.
Most of their fear doesn't come from within them, it's our fault.
Whatever you do is going to be a life lesson and hunting isn't child's play. Help him man up a little, you'll both be further ahead. He'll be fine.
6.5 Creedmoor is an excellent all-around chambering with reasonable recoil. A rifle for life. 308 is an excellent round but will definitely have more felt recoil than the 6.5, I've shot thousands of 308 and hundreds of 6.5. 243 is also an excellent choice. There are more. Give him a DEER RIFLE, not something he can get by with.