The way to get weight on the mare is to wean the foal. At 5 months, he's fine on his own. At 5 months, the milk isn't providing much nutrition either. After 5-6 WEEKS, the nutrients and energy in milk declines rapidly. A mare requires the most Mcals at 1 and 2 months (about 54 Mcals/day) At 5 months she needs about 48 Mcals and the colt needs about 25 Mcals. That's the equivalent of 10# concentrates and one 60# bale of grass hay. Even if she has a round bale in front of her 24/7, she might not be getting enough calories out of it. Her round belly is an indication that it's mature, high fiber hay and therefore lower calorie. I wouldn't up her concentrates but I would offer higher quality hay. Give her a few flakes of alfalfa (10+ pounds) a day. That's an additional 10 Mcals that are easy to get into her. The M&F will be higher in calories than the SC so as long as the colt is on her, keep her on the M&F. When you are ready for her to dry up, stop all grain for a few weeks.
It's a fallacy to think that the mare will naturally wean the foal on her own. Without another foal on the way, she will just keep letting him nurse. I have seen 3 year olds still nursing. Colts are harder on their dams in my experience. Get busy on building someplace to put him. Do you have a friend's to take the mare for a month?
The success in keeping weight on the mare starts in the last trimester. A mare that is a little plump before foaling is the best bet as most mares just can't consume enough calories when nursing. Most mares are going to be down 1-2 points (back to where they should be) no matter how much you try. At 5 months, the colt should be eating 5# of concentrates ON HIS OWN. Every thing he eats, is that much less he takes from the mare.