Hi i'm really concerned on my thoroughbreds weight and i'm in Australia he is aged 19.?

i'm wondering if someone could give me advice on how many scoops of mitavite gumnuts i give him because the dosage that i am giving him now which is about 3 cups is not working at all! also in his diet i am giving him molo mix which is lucerne chaff mixed with oaten and moles i also give him good quality gr hay 2 biscuits a day! please help me i'm very concerened of his weight and the amount i am giving him isn't working thanks emma =]