Chewing the feet can be caused by a number of things. First, we think of allergies. It is a little unusual for allergies to suddenly arise at 7 years old, but it's not impossible. Allergies are very difficult to narrow down. They can be food related or environmental. Sometimes we also have to consider changes in the home, like household products that you use such as cleaners or laundry detergents. Even sometimes your own soaps or shampoos can set of a dog's allergy, especially if they spend a lot of time with you snuggling. You may want to rinse his feet when you come in from outside to remove any possible outdoor environmental allergens.
As for food allergies, have you changed anything in his diet recently? New treats? New food? Different flavor of the same food? All of these things could signal a potential food allergy that has suddenly come to light. Food allergies can be tough to rule out. You would have to do a food trial for at least 8 weeks. This is when you choose a novel protein, something like venison/bison/kangaroo as the main protein in the diet. You would then have to feed your pup ONLY this food for a MINIMUM of 8 weeks before you could know for sure if it was a successful food trial. These food trials can be a little daunting and frustrating for both pets and their people, but they do take time to notice results.
Aside from allergies, licking at the feet can also signal discomfort. If your pup is only licking one foot, or only licking the front or the back feet, this could signal that something is uncomfortable. Check between his toes for any growths, foreign materials, or even matted hair. A Bichon/Poodle mix may definitely have some matted fur between the toes. Your vet or groomer can help to remove these.
As far as supplements: For allergies I recommend the Stimmune and Alge-Ex. The Stimmune encourages the body to respond more appropriately to allergies. It helps to support the immune system and fight recurring infections. The Alge-Ex helps to control the histamine production in the body. It helps to reduce inflammation and itchiness. The Stimmune is more indicated for food allergies and the Alge-ex more so for environmental allergies, but if you aren't sure which is the culprit, both work very well synergistically to knock down allergy symptoms. If you believe the licking might be due to some discomfort, such as arthritis, I recommend a combination of the Old Timer and the Turmeric. Together these supplements help to fight inflammation and soothe painful joints.