It is possible that the bumps your baby has under his testicles are due to diaper rash, which is a common irritation that can occur in that area. It is recommended to apply a zinc-based cream at every diaper change and to avoid using scented wipes or diapers. Make sure to change the diaper as soon as it becomes soiled or wet. If you do not see any improvement after 5 days, it might be a good idea to seek further advice.