Hi Wendy, Welcome to JustAnswer. I am a licensed veterinarian and I'd love to be of help to you and Harry. It is great that Harry is eating so well still because he probably has a very treatable disease. . .hyperthyroidism. Hyperthyroid kitties eat tons and don't gain weight and can even lose weight. They are usually VERY vocal, may urinate and defecate in all the wrong places, may have vomiting and diarrhea, and may overgroom.
To determine if this is his problem, your vet will need to do an exam and blood test. There are other possibilities also, that could be troubling Harry. He could be an early diabetic. These kitties also initially eat a lot, but will ultimately become very ill without treatment and their appetites will wane. This again can be discovered in a blood profile.