Suggest Treatment For The Red Blister In My Kids Chest

My 7 month old son was in contact with a child with chicken pox. 13 days after, a single red blister appeared on his chest. No other spots have appeared since however the initial spot seems to have grown to the size of a 5p piece over a 5 day period. There is no discharge, malodour and seemingly non-painful. He has been slightly unsettled but eating, drinking and no temperature. Visually, it is similar to impetigo... No other members of the household are showing any symptoms. I have this morning applied Clotrimazole 1% to the area. My gp is unable to examine him until Tuesday - do you have any advice?