entirley depends on if he is a good dooer or not, if he gets fizzy on high energy food ect. start with ad lib forage hay/haylage, and work your way up, if he doesnt need condition, just a simple low mollases chaff and balancer would be enough so he gets the vits and mins needed.
My horse is a rising 6yr old TB in medium work and he gets haylage plus a bit of HIFI, fibre nuts and baileys lo-cal balancer and he does great. My mum ex-racer is on the same but instead of lo-cal he gets baileys outshine as he is a tad harder to keep weight on. When my mum got her TB the old owner used to race him on chaff and pony nuts, he would only get a handful of racehorse cubes on the day of the race.
I don't believe a rich diet is good and I think high fibre and oil is the way to go with a good balancer or supplement for vits and mins if necessary. Every horse is different though and you know your horse best