Yeah, 15yo is not old, although domestic horses are often 'retired' due to lameness or body probs by then.
As well as considering doing bloods, consider if he's been *effectively* wormed - depending on what's used, environment, horse's health, etc, normal paste worming may not always be effective enough. Ulcers may also be a problem, especially if perhaps he hasn't been fed properly in the past, had had high starch diet or has been on meds for something. It may also be that he's just lost muscle tone, has a back prob that's caused his topline to atrophy a bit or some such & has nothing to do with feed.
Without knowing your setup, don't know whether he perhaps could do with more or less than he's getting to eat now. Horses should get around 2.5%bwt daily in roughage(eg hay & grass) for maintenance They need to have near constant small amounts going through their system, so it's important to feed little & often, or put hay in a small holed haynet or such, to ensure they don't gorge & then go hungry for periods.
If they need added calories for weightgain, I think there are generally healthier options than grain or otherwise starchy feed, but especially if feeding high starch rations, little & often, rather than only a couple of larger feeds daily is very important, to maximise the goodness the horse gets from it as well as reducing risks of stomach ulcers, colic, laminitis & such associated with high starch diets.
It could also be nutrition as opposed to diet that needs improving. You don't mention any nutritional supplementation & horse's diets are generally imbalanced/deficient in a range of things. For eg. Alfalfa is actually high in a range of nutrients, especially calcium & protein. Too much protein, especially for mature/older horses can be problematic and too much calcium can put phosphorus & magnesium balance out of whack & cause probs. Feedxl.com is a great program/service for diet/nutrition info & sortings.