yes absolutely great idea to do a gentle back & shoulder massage - obvioulsy you need to make sure he is comfortable and the leg is well supported, and be aware that the increase in circulation etc. might affect how fast he absorbs any meds he's on - if you did pre and post op arnica, then hopefully he's not on too much pain meds.
I'd also lie him prone and do lots of work around pectorals, tops of his arms, and shoulders, as they will be stiff if he's using a stick/frame at the moment. Opening up accross the pecs is also really good for posture, as it enables the shoulders to drop and go back. (this also opens up the sternum area which stimulates the immune system - another good thing for post-op rehab.) Correct posture is really important as he rehabs his walking action. Hopefully he's being rigorous about doing the physio exercises: these are vital and will make all the difference to the long term success of the op.
My late Pa had his knee done at age about 66, having gone from running around the common twice a week to hobbling about on crutches as he waited for the op. With 6 months of the surgery he hiked up Vesuvius!
good luck,and keep us posted of progress.