Can you upload a photo(s) of the affected skin to our conversation? You can use the paperclip icon in the toolbar above your message box (if you can see that icon) or you can use an external app such as dropbox.com/ I suspect that a lip fold dermatitis/pyoderma (bacterial skin infection) has arisen in his jowls perhaps after his swimming which kept his jowls wetter longer than usual. Rather than peroxide, please consider the following:
1) Cleansing wipes (i.e., alcohol-free acne pads, baby wipes, chlorhexidine-containing pledgets, other antimicrobial wipes) used every 12-72 hours work very well.
2) Alternatively, routine topical therapy can be used to control the skin problem. The affected area should be cleaned every 1-3 days as needed with an antibacterial shampoo that contains chlorhexidine, benzoyl peroxide, or ethyl lactate. These shampoos can be found over the counter in pet/feed stores, online, or at his vet hospital.
3) Topical application of an antibiotic ointment (over the counter triple antibiotic ointment, e.g.), solution, or spray every 24 hours for the first 5-7 days of therapy may be helpful. The safe prescription mupirocin ointment is available through his vet and should be considered.