Yeti, no contest.

Even my ASR (only 4 inches of travel) is better at soaking up bumps than the 5 inch bikes I tried (including some from Santa Cruz).

The Yeti 575 is hands down the better bike if you want something that can climb but that will soak up the bumps well for a bad back (or any other reason).

I can't believe how much better and more confident my Yeti has made me in just a year. Goes where I point it and keeps me upright.

If you can justify the drive Yeti will happily let him ride one in Golden, CO (their headquarters), or you could call them and see if the demo truck will be near you sometime soon.

If he can, yes, try out a ASR5 as well, but I don't think the 575 will cause any issues climbing (depending of course on setup). It's not a race bike, but it's made to climb efficiently.