Doesn't sound like a very helpful Health Visitor . . . If he does not have a red sore bum often it could be teething rash. (Yes it can effect the botty area, poor babes!) The reason I say this as that is the only time Amy gets it and you know that is what it is as she had it really bad before getting her 4 top teeth, so it may just be that. Sudocreme should help, if not try Bepanthan. (SP?)
Hope it gets better soon.x