Penicillin is typically given one dose a day for three days, but double-check that you're not using long-acting penicillin (it'll say LA on the bottle if it's long-acting). If it's long-acting, you only need to give him one shot every three days, and read the label for dosage. Just don't underdose him, as doing so will just breed bacterial resistance. Although, tetracycline may've been a better choice, considering it's a staph infection. I usually reserve penicillin for respiratory infections and use tetracyclines or other antibiotics for everything else.
A good choice to help dry out the wound and draw out infection and moisture is copper sulphate used as a poultice; it's commonly used in hoof abscesses and works really well. I hope he gets better soon; the wound looks pretty bad, but it sounds like you're being a very good vet for him, and that you're doing all the right things. Good luck!