I think you need to work on movement so it can lower his back as well. A back constantly lifted can be as rigid as one which is constantly dropped.
I would spend little time on a perfectly straight line, instead using very large figures and arcs to help get lateral bend, which in turn helps loosen the back so it can swing. It's only in those softer, swinging moments which may take a few months to get that I start adding straight lines and seeing if I can add some impulsion while maintaining the relaxation.
Hope it helps!