There are some good crssbows out there, but to address this particular problem; the way you have to cock with a rope cocker puts strain on the back and might not be any better for him than a compound, while the best crank cocking mecabism is the accudraw by 10pt and can be used with out any strain on the back thats the reason I have one, if you have a strong preferance in x-bows it can be mounted on x-bows other than 10pt.
Under your circumstances I would also look for a light weight[physical].
Take a look at the Tition[10pt] and Pheonix[excaliber]
Lee