1 year old son with strong body odor

Hi, my one year old son has had a strong body odor under his arms for the past few months. At first we thought it would go away, but it hasn't. This smell gets better right after a bath, but within an hour of his bath he smells again. He smells like a grown man. The doctor has always said my son is small for his age (about the size of a 7-8 month old.) He still doesn't quite fit into 9 month clothing. I am wondering what this smell could be. Also, could the smell and his small size be related?