Of the many things I read about this, one if the best flea deterrents is to put one table spoon per forty pounds of the dogs weight of apple cider vinegar in one quart of water for him to drink. Once it gets absorbed into the skin, the fleas won't bother the dog any more. It does work ! There are some others on this site I've used before that you might check out.
6 Ways to Naturally Prevent and Get Rid of Fleas on Dogs
everydayroots.com/flea-remedies
