I wouldn't be suprised if your horse had some atrophy- he was starved and I am sure not worked. Here is a link to some pics I posted of my guy a few months ago, he is even fatter and more muscled now. before and after (feed= good)
Mine didn't have nearly the neglect yours sounds like he suffered but the amount of muscling I see in him now versus when I first got him is profound, you'll be amazed what good feed and when he's ready, gentle work will do!
As for signs and symptoms, they can be pretty non-specific and diffuse, like any neuro condition. This is partly what makes diagnosis so hard. He might stumble just a bit or he might have tremors, or in drastic cases seizures. I still advocate a vet check up for general health but honestly as far as EPM just watch him, as it sounds like he is not outwardly symptomatic. I would put money on this horse just being a severe malnutrition case based on the history. By the way, how about some pics of your new boy??