If the mare is struggling to hold her weight then she has rights to, so I would take her away and leave him with the herd, he will probably never notice that she has gone.
I have weaned at all sorts of ages, I always abrupt wean, because to me it is far more cruel to drag it out, both on Mother and colt. I've never had bad issues down the road. From mommas point of view if you wean, you can keep her on a lighter diet for a week or so until she starts drying up and then start increasing her feed so she is at a good healthy weight before the winter. The colt is not the only part of the equation here.