@ddljohn-- Baking soda works best when the clothes are wet. So I just add baking soda into the washer and that seems to do the trick. Baking soda as a paste doesn't absorb if the clothes are dry.
Something else to keep in mind is that the longer the dirty clothes wait to get washed, the harder it is to get the odors out. Wash your clothes as soon as you get a load ready. If your son's clothes are the only ones with the odor problem, you might want to wash his separately. Soaking them in warm water with baking soda as soon as he takes the clothes off might be a good idea. That way, the odors won't have a chance to settle.