Before when your baby is hot or when the weather is hot do you notice any rashes?
Under what conditions have you seen similar rashes before?
If the weather has been hot before and rashes as such appear it could be a heat rash. Usually it would subside if you cool him.
If the rashes has appeared before under other circumstances, it could be eczema (aka allergic rash). It could be anything. My baby's allergic to many types of food like egg and chicken and carrots.
If you've not introduced any other new foods recently, and ruled out heat, then it could most likely be a drug allergy. The rashes would take a few days to subside.