When X was little (3ish), there were plenty of times with PT and OT when he would be silly or uncooperative. Those therapists let him pick a little treat at the end of the session if he did a good job and they found that if they either said he wouldn't get a treat or that they'd have to send Mom out of the room if he didn't cooperate, that he would straighten up.
Now he's about to turn 6 and is working with a new in-home PT and a couple other therapists. PT has always been the one he's most resistant in b/c he has hypotonia, mild SPD and just doesn't enjoy moving around or challenging himself physically all that much. I think he feels like he might not be successful so he can't be bothered to try. It doesn't help now that he has therapy 2 days/week (just moved down from 3) right after full-day Kindergarten. His motivation seemed to be improving recently though; he told his PT that he wanted to get stronger and he wanted her to help him get stronger and be able to do things other kids his age can do. (He recently scored at 45m on his re-eval; he is 71m old.)
Today he told his PT that he wanted her to pack her bags and leave and refused to work with her anymore. She is very experienced and good with him and she tried to talk through it with him and gave him chances to change his mind but he was basically just telling her to leave, that he didn't want to work anymore, and not taking it seriously. For context, he's been doing very well in Kindergarten and takes great pride in coming home on "green" every day but cooperation with PT is very hit or miss. How do/would you handle this?