Ooh owch! It looks like it's possibly an abscess that's burst, it'll need cleaning by a vet too.
Try and get a vets appointment as soon as possible.
If it is an abscess it'll need flushing out 2 or 3 times a day (abscesses heal from the inside out) he'll also need some strong antibiotics Zithromax is used with good success rates in the UK for abscesses, he'll probably need to be on that for about a month, if any infection is left in the wound it'll only break open into another abscess in the future!
one of my guineas had a facial abscess too,but hers was lanced then sutured open to let it drain (marsupialised) , daily i flushed the wound with baytril, she had Zithromax orally &some loxicom for the pain.
make sure no flies can get near his face, if you could put any hay on the floor for a bit instead of above him,just to stop any bits falling into the wound.
@furryfriends (TEAS) is an abscess queen,but here in the UK it's nearly 1am,so the health & illness members (volunteers) might not see your thread for a few hours.
He's a handsome chap,
fingers crossed for a speedy recovery,let us know how you get on at the vets. Xx