How do you deal with an elderly man who still keeps self medicating himself?

Every ad ;tv or mail he tries. The last one had shellfish enzyme in it. His med list lists shellfish as an allergy. I personally think it's the iodine ,since he has thyroid problems (he has eaten clam chowder w/o a reaction.) Any way this has been going on for the last 2 years. His son is getting tired of it too. But because he is somewhat with it (he has 2nd stage Dementia) there is not much we can do. Any ideas? This is getting to become a problem.