Is he teething Amber? That is the only time DD has got a rash. If the rash is bad, i use a nice thick healing/barrier cream and put a microfleece liner in the nappy. Go to your local spotlight, get a bit of microfleece. (not polar, must be micro) and cut it in a strip to fit in the nappy. Protects the nappy but still keeps the bum dry. Also give him as much nappy off time as you can manage.

If the rash persists and it is not teething, i have heard of babies developing a reaction to certain fabrics. Generally synthetis fabrics like suede cloth, microfleece etc.

In regards to cream, i really think different things work for different people.
Many people love Moogoo as you can use it with MCN and don't need a liner. I find it great, except if the rash is bad then i find i need something 'stronger'

Sudocream doesn't work for us, but many people swear by it. Penaten and Bepanthem work best for us. It think you need to try one out and see what works.

Oh and one other thing i have done in the past is use a diposable overnight and REALLY slather the cream on.

Hope it clears up soon!