Don't quote me on it, but I'm pretty sure CP is contagious from a few days before the spots appear and until they have crusted over? Haven't had any recent experience with it though...... it might pay to get him checked out by your GP but perhaps ring them first to let them know/see what they suggest. If it were me I wouldn't be going out in public if he's contagious as a courtesy to others.

Poor little guy, whatever it is I hope it clears up soon and isn't too rough on him, how sad if his birthday celebration has to be toned down