Hello and a warm welcome to HCM,Calamine
lotion is used worldwide and is one of the safest lotions available although you should be careful that your baby does not accidentally eat or lick it.
You can apply clothing over the area but the recovery is sooner if kept open
hope he recovers soon
Dr Sriram Nathan
ps: meanwhile try finding out what could have caused the urticaria
, it could be anything like mites, insects, some dirty mat etc