It could just be the baby shampoo irritating his skin. Some dogs are more sensitive than others to human products because the ph of their skin is different from ours. I agree about trying a doggie oatmeal shampoo. Once a week might end up being too frequent for your baby but most of us bathe our dogs once a week without any trouble. One of my dogs has very sensitive skin so I don't use the blow dryer on him. Good luck!
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226