DD1 Has a sore bottom at the moment too, was screaming all the time with it. ( i think it is from something she has eated though & just need to narrow it down to what her nanna gave her tomato's & i think it is the acid in that) In the end, I have put her into a salt water bath & honestly, it's the 1st relief she has had in the last few days. Now with keeping on top with the pawpaw cream (or try the cornflour in your case) and regular salt water baths, it is almost gone today (yesterday it was bleeding raw!)
Poor little man, I hope he gets better quickly!