Thanks to gedabrcaer for this Frugal Baby Tip!
If you are still breastfeeding, you could try breastmilk. You just express it directly onto the rash. It is more comfortable for him, since there is no rubbing. Breast milk is naturally anti-bacterial, anti-viral, and rich in Vitamins A & E (among others). It should quickly heal the rash unless it is yeast.
It won't leave build up on your dipes. Before we discovered the breastmilk solution we used and loved Weleda nappy change cream. I don't know about buildup, because we never had to use it long enough for it to build up.