I have a human friend who eventually (to his surprise) outgrew his heart murmur. He always figured it would keep him from getting drafted if the military ever went back to that method, so was quite surprise when in his late twenties his heart doc told him the murmur was gone! We can hope for this for Franklin, one never knows!
I am so sorry!! There's nothing worse than finding out your cat has HCM (hypertrophic cardiomyopathy) and there's nothing that can be done. All you can do is love on him like no one's business, give him anything and everything he wants. You never know, he might surprize you and live a long life. Just let him know how special he is.
You'll be in my thoughts and prayers.
I'm awfully sorry to hear that Franklin has this condition, but his prognosis sounds as good as it can be.
I hope he confounds all the doctors!