Hearing loss?

It appears Harley isn't hearing me very well. I've noticed it when I call him in from the back pasture. I use a cow bell, but it doesn't seem to work unless he can see me. Yesterday, Kodak came without Harley. She then went back to "fetch" him, and they both came. Then, this morning, I walked out to check on them, and was about 30 feet away from Harley, calling his name, but he had his head in the tall grass and didn't pick it up. Until suddenly, he saw me, spooked and bolted. He stopped fairly quickly though, realizing it was me, then walked over to me. Kodak, who had been standing a few feet away from him, heard me the first time I called to them.

Not sure to what extent he has lost his hearing, or when. I'm going to do some experiments to see if he can hear at all. But should I be doing anything differently? Can I expect changes in behavior? I suppose I should make sure he can see me when I approach him. In terms of riding, we don't use verbal cues with him, so that shouldn't change much. He's a very visual and tactile horse (maybe he's been using this as a coping strategy), so we communicate that way.

And should I get the vet out? I don't think he's had an ear infection or anything like that, because I handle his ears a lot to keep them trimmed during show season and he doesn't react at all.