I have a similar bred horse who was about the same weight as yours, but he is 16.2hh.  To be honest I think the reason we have managed to get him slimmer is through exercise which you can't currently do with yours.  My horse gets turned out during the day in a field of poor grass and only gets haylage at night, again haylage is not ideal if you have laminitis.  Why not email one of the feed companies and see what they suggest, and also contact the laminitis clinic for advice, they have been really helpful to me in the past.

Hi, I have a clydesdale x cob, who is also 15.3! He is a bit of fattie too, but has so far luckily not suffered laminitis. He weighted 706kg, and with alot of work we have now got down to 603kgs, he is still overweight at this but is doing lots better. I totally sympathise with you it is hard work to get the weight shifted and its took me six months so far!