His whole face looks swollen on that side. Maybe he got his fly mask caught on something, pulled away, and when he felt he was still attached, thrashed his face into it and that's how he got the gash and the reason his eye and face looks swollen. There really is no telling with horses. They will find the one thing you didn't think of to hurt themselves on.
Thoroughly clean the wound, spray vetericyn, and if no progress in the next day or so...call your vet to check it out. Also ask your vet what else you can do if he hates wearing a fly mask.
Yesterday is history, tomorrow is a mystery, and today is a gift-that's why it's called the present.