Why doesn't he want to wear his coat?

Hi, my little boy is 5 and a half, he was diagnosed ASD just before his third birthday.  For some reason he has decided he doesn't want to wear coats this winter. Last year we had hardly any trouble with getting him to put his coat on to go outside.  When he first started refusing to put it on we thought maybe it was too small as he has grown alot lately but we are having difficulties with the new coat now.  It is a big cause of stress for us and him and I would just really like some advise.

Thanks