I have a similar problem with mine at this time of year though he's a pony and not as old as yours. As soon as we moved onto last year's haylage in January and the hours of turnout were restricted a bit he gained weight. We like you have too good haylage apart from the odd bad bale and we're not allowed hay so I've had mine on a weight loss programme for several weeks and not seeing much shift yet - but bear in mind it does take time, a long time for weight to come off, once on. I've had mine with no stable rug for weeks (apart from at the moment when it has dropped to 0 overnight again) and he's out in a lightweight no fill turnout. Lots of trotting when we're out hacking too and our field barely has any grass as is mainly mud but I'm still having a battle so I sympathise!
Beyond what yours might need foodwise for his age, have you tried Top Spec's Top Chop Lite - it's a totally unmollassed chaff which someone on here recommended to me as I just needed something to put my pony's supplement in. And you can use it as a partial hay replacer too as fibre. Or a balancer like someone else has recommended just so they're getting the vitamins and minerals they need on top of their forage. Best to always speak to the feed company's helplines for specifics on their products though.
Is spreading out his hay ration through the day an option? i.e. hang a haynet/put hay in paddock? Cut out hard feed and split his hay half and half with straw. It's only temporary then when he loses his excess, give him a balancer with some chaff.
Does he really need any hardfeed at all? My arab (believe it or not a good doer) only has a handful of happy hoof to put a supplement in. I often get other owners thinking I'm Dr Evil but he lives off winter grass and hay perfectly well - I'm sure you could cut down his feed to just a handful.
Thanks guys for all your suggestions, it was the 16+ doing all the fattening up!! He has been off it a week now and is already dropping weight slowly, according to my tape
To those who suggested less haynets....... I have ALWAYS fed 2 haynets and ALWAYS will, everyone to their own preference but i would rather see hay on the floor in the morning than not a scrap in sight! Like i said before, its only recently we have been struggling with his weight and now i/we have solved the problem!!
Thanks to all who told me to get him off the D&H 16+, after you said about it i had a read on their website and my jaw nearly hit the floor!!!!