One of my cats gets this everywhere on her body, and hers is a flea allergy, so it could possbly be that, you just have to deflea all the time.
I would wash it in a betadine solution, as it is an antiseptic.
I'm not sure what season it is over there (here we are in spring, so every allergy is popping up here) but it is possible it's an allergy to something.
How long has he had them for?
Where abouts on his body are they?
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms