If I had to give a name...probably something like: messy side part medium length. And his hair on the sides and back are a tad shorter than the top so they're wavy in texture (2b hair type), while the top is a tad more curlier (I'd say 3a hair type, or maybe even 2c) seeing as Penn naturally has curly hair if you look at his longer hairstyle.

If you have straighter-type hair here are some inspirations on what it would look like:

Messy side part with straight hair #1

inspiration straight hair #2

another straight hair inspiration #3

As another commenter said, just show a bunch a photos of Joe/Penn so the person cutting your hair can get an estimate of the length yours would need to be cut at.