I need to put weight on my thoroughbred?

I have an 8 year old thoroughbred who raced until 8 months ago. He gets 2 scoops of pasture mix, 2 scoops of sugar beet, and 2 scoops of oats, everyday divided between 2 feeds morning and evening. At the minute he is on 24 hour turn out, and when he is in he has endless supply of hay and haylage. He is ridden 3-6 days a week, depending on my work load. Other than bringing him in from the other horses during the day or at night and give him endless hay, I am a little unsure what to do. I have had him work counted, and he is completely clear. When it is not warm he has a rug on. HELP??!!