Well..... I've had this issue! but with a quarter horse appendix. So here is a little what you have to do, since horses are grazing animals, you have to keep him on some pasture, maybe a sections where not much is growing.

remember if you want to reduce his feed, you have to do it a little at a time, I usually took one small hand full at a time every other day.

exercise, but remember! not too much!

You don't need to get his weight down it is good for a cob to be fat because when you start really working him all that fat turns into muscle and if there is no fat there is nothink to turn into muscle so dont get him to lose weight just when the wheather is nice do a 6 week plan to get him really fit and your cob will be very muscular.

i should know my horse Solo roxs is a cob

Sounds like your nearly there. If you're just feeding forage then consider giving him/her a mineral supplement. I have a 14.2hh heavyweight cob and she is fed 2 cups of baileys lo-cal per day so i kno she's getting everything she needs but not the calories. Well done for looking after the weight all the best.