He is really cute!! Love his colour!
I wouldn't feed him any less, I would ride/lunge etc. him more. Bonny is a bit overweight and I try to "work" her as often as possible, sometimes twice a day.
Eg. my parents live near the stable where I board Bonny. On my way to their place I quickly stop off at the stable and lunge Bonny or have her run free in our arena.
When I go back to the stable at night I take Bonny for a ride.
She doesn't just only lose weight because of this, she also stays in training which is very important to me as she is 20 now.
I also have a girl who rides her on the weekends, when I normally haven't got a lot of time due to work etc.
You can check yourself if your horse needs to lose a lot of weight. When you can't see it's rips but feel them without putting a lot of pressure on your hands, your horse is in a perfect shape.
The harder it is to feel his rips, the more weight he has to lose