Firstly, have to say I got a laugh out of your signature line!
Yes, IMO it is a good idea to feed a 'ration balancer' or otherwise supp for balanced nutrition. Yes, there are a number of good ones out there that are grain/molasses free. But generally they are low dose, so won't add that much in the way of calories.
Have you had him tested for cushings or any such? I'd be inclined to, if he's old, getting a good diet & not putting on. I'd personally want to consult an equine nutritionist, or use a service such as feedxl.com as well.
How long has he been on the Founderguard? I wouldn't personally use this & it's possible this is causing weight issues, as it's an antibiotic & horses digest their feed in the hind gut predominently via bacteria. Also I'm not sure founderguard is very effective at avoiding insulin resistance caused founder(grass founder for eg) - thought it was more for combatting hind gut acidosis due to bad diet & feeding practices, such as grain rich meal feeding.
A whole acre plus that much lucerne should be more than enough for him without added calories, so something's not right.