He is 1500 lbs, 14yrs, & it depends, usually 6 days out of the week he gets ridden or lunged, he is in the pasture in mornings.
that's a BIG horse. Is that an actual weight measurement or a guess?
I'm assuming he is not in heavy work tho, correct?
If he actually weigs 1500lbs, then 15lbs of hay isnt enough.
If you increase his hay intake, he will most likely gain the weight you want. Horses need a MIN of 2% of their body weight in forage (hay). He should be getting 30lbs of hay MIN per day. Free choice would be even better, especially if you're wanting hinm to gain some weight.
If I were you, I would increase the hay and I would also back the enrich down to 2.5lbs per day which is about 6 cups.
If you still don't see the weight that you want after the hay increase, then add more calories in the form of alfalfa pellets or cubes, rice bran or beet pulp.