For dd, baking soda didn't help. In fact, she screamed when I used it and I had to run a new bath.
You could make your own oatmeal bath with ground oats. Much cheaper than the brand stuff. Just put it in something like a piece of hose or something and tie off, then throw in the tub. That way the ground oats don't clog the pipes.
Also, Rooibos tea works wonder on rashes! Brew some up and add it to the bath. You can also use it to wipe his bum after changges. It really does work great.