It does look and sound like classic velvet, but he may be fighting off a secondary infection as well (extremely common with velvet due to numerous insertion points) and of course antibiotics would not help in your case of achieving immunity.
My suggestion would be a quick freshwater dip if you can catch him to help reduce the number of parasites, up his feeding and soak his food in vitamins and selcon, and if you can without hurting other livestock shorten your light cycle a bit.
My suggestion would be a quick freshwater dip if you can catch him to help reduce the number of parasites, up his feeding and soak his food in vitamins and selcon, and if you can without hurting other livestock shorten your light cycle a bit.