Ohh you poor thing!!
Did he not have it this morning?
If so I wonder if it is more than nappy rash and maybe an allergy?
If you think it is nappy rash try Bepanthen or Curash as they will both soothe the pain!!!
Bepanthen is great for claering anything up quickly, Curash has a zinc base so it dries things up and sometimes can cause more discomfort if it is already dry down in that area!
if you are really concerned go to the doctor in the morning just to get it checked out!! For your peace of mind!!!