There's really no need for bubble bath if it's drying out your son's skin. After 1 winter of severe eczema, I threw out all our bubble bath. We now play in plain bath water, and we do all the washing at the very end (Burts Bees or Aveeno products). Then, as the water drains, I have my daughter stand up and I spray her with the handheld shower head to get all the soap residue off. Finally, I use Burts Bees apricot oil on her "trouble" spots - knees, tummy, elbows. Also remember that kids this age really don't need a daily bath - 2-3x per week should be enough to keep him clean without drying him out.
And don't forget that skin and hair health is directly linked to nutrition - make sure he's getting plenty of fluids and the right amount of healthy oils. Add a teaspoon of flax oil to his oatmeal, make sure you cook with olive oil or another healthy oil. Seriously, this really does make a difference!
-K.