Your herd dynamics will be the same when he is reintroduced to his mother physically.
However with the hay and grass situation you are dealing with remember that foal will be getting some of his meals for free from his mother.
Once you start weaning him he will be competing for grazing with the other horses.
If I were you I would feed the mare more grain to reduce her weight loss and allow him to nurse a couple of more months to stretch your hay and grass out until the fall. A complete feed or a senior feed might be better for her and feed race bran with it. Or go the expensive route and get Omolene 300 it already has amplify nuggets for weight gain added.
That foal needs certain nutrients in order to develop and grow to his full potential. Shalom