My nephew got a rash (like thrush) on his penis and the gp told my SIL to use canistan to clear it up (as it was fungal). So I would think canistan is fine.

If it is from an irritation (rubbing against his nappy) then vaseline is what they told us to use on DS after his circumcision to stop the pee irritating his penis and also to stop the nappy sticking. Not good for it though if he has a fungal infection as that can just help the fungus grow and canistan is the way to go.

Hope that helps and the rash clears up soon