Hi Kate671
50 x a day! that is really dreadful. Poor guy. I can understand it is really difficult to do much mobility and balance wise if his shoulder is just so unstable.
Is there a pattern to the dislocation (direction? dislocating movement?) or is it in more then one direction? If it is directional thinking of an immobilisation that prevents that direction may be helpful. I think you really have to come up with something that immobilises the shoulder so he can move his body freely without the shoulder dislocating.
I would think a Bobath sling would not give enough support. Possibly you could try to taping his shoulder at the beginning of the session with the aim of compressing the head against the glenoid fossa. and prevents gliding n the direction(s) it dislocates in. if it is dislocating inferiorly then taping or a sling that stops inferior subluxation may be worth investigating.
I know you are not treating his shoulder but FES to the total rotator cuff at the same time might to build up some strength may help matters. He was obviously relying very heavily on his rotator cuff prior to the stroke to keep the shoulder intact.
Other then that I don't know what to suggest.