Shoulder - post-op weakness/instability

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My partner - 42 y.o. male - had a shoulder arthroscopy 2 weeks ago. Previous to that he'd been almost immobile for about 6 weeks due to pain. Biopsies show he has Synovial Chondromatosis but as they cleaned everything out while they were in there he's not in the same pain as he was pre-op.

The external swelling has gone down but he's still a bit sore.

After a few false starts he's finally got the pain under control and has been having lots of physiotherapy. His range of movement is massively better but his brain just doesn't seem to want him to do certain things.

For example - one exercise he's been given involves putting his 'bad' arm in a carrier bag (so it slides) and laying on the bed and 'lifting' his arm out to the side - like a 'snow angel'.
He can't do this unaided at all yet when I 'assist' I am really not assisting at all - I put one finger in the palm of his hand and then pretty much just follow his arm out to the side. Why??

We think that there's must be some kind of 'mental block' going on because his body knew it hurt before so just won't let him do it anymore.

Does anyone have any idea what's going on? Is it really likely to be a brain thing - does he just have to remind his arm how to do this movement and prove that it doesn't hurt any more?

Also, he's very unstable and wobbly if he lays down and makes a 'bicep curl' type movement - his bad arm seems to want to 'flop' toward his body in the first part of the movement.

Is he just being an impatient patient and expecting too much, too soon?

(He has an appointment with the consultant next week but we're just intrigued, more than anything else - hence the question.

Thanks.

T