If he is "hearing" with his cochlear, you don't need to cover your lips. Do hearing people do that with their hearing children?
If not, then there's no need to. If he really struggle to listen to the point that you have to cover your mouth to teach him, Just stick with sign language. People in my generation of hearing aids did the same thing to us. They covered their lips so we could be trained to use our hearing aids. It worked in the short-term if we are really focus on it, but in the long term, it doesn't work.
Cormac in no way needs to lip read. It;s not like we walk around with our mouths covered. But when we are testing him we like to make sure he isn't "cheating" since he is such a good lip reader. Parents of those with hearing children don't have the same issues. They KNOW their children are using their hearing because a hearing child doesnt lip read. He uses his hearing to have conversations. He uses his hearing alone in a mainstream preschool and doesn't miss a trick.
I have heard of cued speech but haven't looked into to much. It sounds like an option. We so rarely need any assistance in communicating it hasn't come up to often.
Venemous... I know. I honestly can't believe the things he remembers. He is like a sponge. Your Peas sounds just like him :)