Harv's hind legs

Yesterday I took Harv out for hand-grazing. As a retiree, he is allowed to roll when he wants, including when he is in hand. He rolled in a particularly luscious patch of tall grass, and normally I enjoy his enjoyment. He has been having minor issues getting up in that he doesn't get up as easily as a young horse would. But yesterday,  he had trouble getting down. He gets part-way down and then falls to the side the rest of the way, and he hits pretty hard. He rolled fine, but getting up, he sat like a dog for a long time. It took him three tries to stand and it's clear that his hind legs are not very useful in the process. He's throwing himself up on his front legs. 

He's overdue for a trim, so I'm having that done to see if it helps. I may try more bute or some Previcox short term, to see if that helps. But I don't think it is a pain issue so much as neurological. Nothing else to do but be watchful.