For a short term solution, you might want to try this: get him an inexpensive pair of shooting glasses and put a piece of translucent tape on the inside of the glasses to prevent his dominant eye from seeing the sights. (This is something like putting a patch over his dominant eye but doesn’t strain the eye so much.) You can use expensive shooting glasses if you want to, but someday you’ll have to clean that sticky tape off the lens. Don’t put the tape on the outside of the glasses or the eye may see a distracting reflection of itself.
Be careful the shooting glasses don’t mess up the seal of his earmuffs too badly, esp. if he also wears regular glasses. It’s probably a good idea to double up on ear protection, anyway – that is, wear both earmuffs and earplugs. With a rifle it’s not uncommon to accidentally break the seal on your earmuffs when you put your face on the rifle stock.
I just bought an Armalite M15T4 myself. Absolutely great little gun!! Incredibly loud, however. Admittedly, I’ve got a shorter barrel than you and I’m also shooting indoors, which you probably aren’t doing. The first time I fired it I genuinely thought I’d forgotten to put my ear protection on – and I hadn’t!
Good luck!!