I would start him on activated charcoal. Mix it in honey, applesauce or a soft food or liquid if he won't take it in capsules. It can be helpful to put it in a covered, opaque glass and have him sip it through a straw. (The black color is rather strange. It is a bit gritty, too.) It comes as capsules and tablets, too. Give three times a day. Start with a teaspoon each time and increase the amount. Always increase water intake and fiber to push the charcoal through.
Second, I would add lots more soluble fiber (beans, applesauce per Karen Hurd) to pick up toxins.
Chlorella is good to minimize detox symptoms. Use a good brand like Nutricology King Chlorella. Try 6-8 tablets per day. More is ok if he tolerates it well.
Consult a pharmacist about all the meds ds has taken to see if any of them cause GI distress.