When my filly was that age she got a small scoop of sugar beet pulp and a small handful of D&H foal mix (not stud mix, foal mix specifically just for foals, you  might need to order it in), and haylage.  She was weaned at 4 months.  She was fed that mix in increasing amounts as per her age until she was 2, then she went onto sugar beet pulp and alfa a oil.

My 7month old gelding is still suckling his mum a little bit when they are turned out. But he is fed Alpha A  chaff ( yellow bag), Spillers Stud and youngstock mix and a glug of vegeatable oil. He is been on this since he has been able to eat from his mums bowl. He has about 3 sl;ices of hay over night and 2 feeds a day. He is in his own stable next to his mum. He has grown well and is filling out nicely.