If .223 isn't the 'right' caliber to use, then sure. If that is permissible for taking deer, just build it in 5.56/.223, run Barnes TSX bullets, and drive on. If you want to make it a BLK, go for it - the impulse between the two isn't very different, and since you're handloading for the BLK you'll be just fine.
As far as making it more tolerable for him to shoot, ironically making it heavier is probably the answer. Put a bipod on it, an optic, hell, throw on a flashlight with no consideration for weight - stuff some spare parts or batteries in the stock and/or grip if the recoil is an issue. A 10lb AR doesn't kick much at all, and if you can set him up shooting prone (assuming climate/location isn't restricted to elevated blinds or anything - I have no idea how it is in your neck of the woods) off a bag, that can help even more.
عندما تصبح الأسلحة محظورة, قد يملكون حظرون عندهم فقط
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Semper Fi
"Being able to do the basics, on demand, takes practice. " - Sinister
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