Perhaps the stock on the 1100 doesn't fit him properly - that will make recoil feel worse than it would for someone where the stock does fit. Benelli makes a LH gun, but they're not cheap - for the price of one of them new, he could get a nice used O/U.
Personally, I would find someone who knows how to fit stocks. If you're in CA, Dale Tate:
Ione, CA 95640, (209) 763-9040.
www.angloamericansport.com ...