Just use a mount that will place the scope just high enough so it does not hit the rifle and will allow enough forward mounting ability for proper eye relief.
If he is trying to fight the aging eyes anything less than 4x won't be much of any gain depending on his eye sight and Rx.
One of my favorite set ups on one of my ARs
Leupold MKII IMS 1" scope mount
And this scope
https://ads.midwayusa.com/product/1...old-_-141511&gclid=CPOAvLrs6dICFVyBswodGE8D8A
Compact simple and works great for me.
It's the simplest set up I have and works good out to 200 yards. Even on 2.5x the front sight is just a slight shadow. If your looking at the target and focusing on the cross hairs it's just not there.
There are many options for mounts and scopes.
My other scopes sightron 8x32x56 and it's a beast on top of a AR. Mueller 8x32x44 "tactical" only thing that makes it "tactical" is the reticle.
Also have the Bushnell AR 223 scope for under 200$ it's been very good. Has held zero although after using the sightron and leupold when I use that set up I see and feel why it's 200$.
I use a particular ammo for each set up and each has its own particular use.
The leupold is set up if i ever get out to do some hog or coyote hunting. The sightron is for 600 yard prone shooting. The Mueller is for 200 yards with my 22lr AR. The Bushnell is zeroed to the M193 I have the most of in the same lot# so I do not need to adjust me zero all to often.
