Oh no. So sorry to hear that. If cancer is like how humans handle them, then good food and exercise should do some good, although not guaranteed. Regular checkups with the vet is a great idea as well. Maybe giving him supplements too, but I don't know which ones at this point.
I can't even imagine how you're feeling. Cancer's a scary thing. Please remember that although it's hereditary, it doesn't mean that Lexter's going to get it. We can only do so much to prevent it, including praying and enjoying every minute with him.