you have not described the type of rash
, if it is itchy.
any other manifestation he has like fever
have you recently introduced any new food , food allergy
could be one reason, you giving same food again again would have increased the rash. other possibility is allergy to any drug if he is taking. allergy to a garment, it could be a manifestation of atopic dermatitis
. all these are few possibilities.
if you could have described the rash we can reach to some conclusion
try to rule out any of following