Congratulations on your new baby! I bet he is adorable. The weaning process will depend a lot on the baby and your mare. Some mares are no problem to wean from their babies and others will do anything to get back to them. We usually wean ours around 5 months or so. My mare is so attached to her babies that it makes it very difficult to wean the babies off her. I just about have to take her or the baby off the farm for a couple of months. Be sure you have a well behaved teacher horse for baby to go with to replace mom when you wean and keep saftey in mind for both baby and mama.
I have to agree with sweetwaterarabians and says it depends on the mare and foal. Common age I wold say is six months, but I have done it as early as five and on the other hand still have an almost seven month with momma still. She isn't producing milk anymore but I still have got them together..... I'll be weaning them soon though.