My mare is stll feeding her 9 month foal - we'll get around to weaning him some day but there's no hurry.
Merlot dropped weight at around the 4 - 5 month mark but she has regained it now and looks great. (you can see pics of both of them on The Zephyr Files right through from his birth till now)
Regarding feed - keep it simple.
Merlot gets, speedi beet and copra soaked with whole oats, linseed flake and oaten chaff. To this she gets 2 Tbsp SALT every day (vital for any horse) and a top quality vit and min mix. When she dropped the weight, we added amino acids which have made a huge difference to her topline.
I am not an advocate of early weaning as I think the foal learns so much from his mother, but I understand your concern about her weight.
Finally don't despair if she is a little ribby as this is better than being too fat and she will, once he is weaned pick up again
Hope this helps