OK, so your first post should have said:
horse is on ample pasture all day, and ad lib hay overnight. And not gaining weight.
I think you would have gotten different responses.
That said, I have to question the “loads of grass” in only 2.5 acres of pasture. 2 horses, 8-10 hours/day, presuming all through the growing season (spring to fall?) - I would find it hard to believe there is still a lot of grazing left. Have you walked it and looked? Obviously everything is driven by climate and soil, but I would expect this pasture to be pretty threadbare by now, which is normal. That’s why I take mine off the pasture so they don’t ruin it, and put them on hay 24/7.