We have no idea what Chris was doing to the wound between when it first appeared on 15 July and when the photos were taken. We don't know whether he just made up his own care routine, Googled advice on how to care for it, or what. We do know that he doesn't claim to have seen a doctor until several days after its appearance, though.
As far as "surely he wouldn't" goes, there are plenty of videos on youtube of people doing "self-surgery". He could definitely have come across those if he was after information on wound care, but if he didn't initially recognise it as a wound then he had no reason to go looking for such information.