Hi, I wouldn't call 11yo middle aged these days, he's still a spring chicken! And while TB's, esp as often OT or performance horses, managed & fed, are commonly 'hard keepers', it's about that management & feeding, not so much about the breed.
Firstly, if the horse isn't getting free choice grass hay, that's where I'd start if there's not ample grazing, of if he's locked up at all. Then I'd provide him with a good supplement/ration balancer that will give him the nutritional balance he needs.
I'd ensure his teeth were in good order, he was wormed *effectively*(sometimes owners can be diligent but regular wormer isn't cutting it - fecal tests may be in order), and I'd be considering ulcers/gut damage, and putting him on a gut support product(I like herbal for that).
Then, if after a bit(he won't get better in a week or 2, regardless) he was still doing it hard, then I'd consider adding some more calories in the form of 'hard feed' of some kind, of which I'd keep low NSC/grain free if poss. If you add oil, coconut or otherwise, ensure you start with very small quantity & build gradually, over at least a few small feeds daily, as horses don't naturally have the enzymes to digest pure fats like that.