My guess was heat rash when it appeared just over a week ago, when it was really hot.
however it’s not gone since, even on the days/evenings when it has been cool. It definitely looks worse when he is hot and his skin in redder. It’s only on his trunk and slightly on his upper arms, in the areas that would be covered by a short sleeved vest. Baths don’t have any affect on it. Totally well in himself, no temperature
Any ideas?