Talk to him. Be honest. Let him know you love him & aren't looking to replace him, but you need more. He knows he's small. If he cares for you, he ultimately wants you to be happy and satisfied.
Once you've talked, don't expect an answer right away. He'll probably need some time to process. Show him this website so he can read and learn.
The first step is to start the conversation. Nothing gets going without that first push. Be brave. Go for it.