I don't think it would hurt to try regular eyeliner, I don't see how anyone would know the difference with that at a show unless you made him look like Tammy Faye Baker LOL (google her for a pic youngsters) I would think that would help him a lot, in fact. Just make sure that you keep the sunscreen going. You could probably try the eyeblack stuff too for when you are riding at home and whatnot. You might even take the eyeliner and/or eyeblack and put it somewhere else on his body for a day before you put it on his eyes, to make sure he wont have a reaction to it. Somewhere that would considered a sensitive place I would think.
The eye tattooing - I am not sure on the exact price, but I don't think it's too expensive. My mom was going to get her paint filly done, but then opted to just let her be her.