I really like the weathered look you've given to his face. It shows a lot of attention to detail. A couple areas of improvement are his chin where he's got a bump, and either it's way the lighting is or his facial hair is very light on the right side of his face. I'd also like to know if you're going to continue drawing the rest of his outfit or just leave it as is and focus on the face details? I think if you did choose to leave it out, Percival's expression would still be striking and the chainmail might distract from that.
Other than that, it's an excellent portrait of Tom Hopper as Sir Percival. Nicely done!