My son had same experience before, you can try don't use any diaper, carry him for we-we, may be every 30 minutes, after 3 - 4 days will be much better.
if you still wear diaper, then use warm water wash his pat pat when you change diaper everytime and then put some cream.
Better do not put any cream even vaseline. Every time use Cotton pad w/ warm water clean the bottom. Change diaper frequently in order to keep the baby's bottom dry is the key. U will see the result in 2 days.