I found that in certain circumstances Henry would still mouth even up to about 18 months of age. His particular crazy place was on the sofa. He’d come up for a cuddle but would very quickly do what we call ‘go curly’, when he’d twist and roll onto his back, flail with his paws and nip. He really wanted a cuddle, but just couldn’t seem to stop the nipping. If he started, I’d get up and walk away a few steps. He’d very quickly jump up and follow me. Then I’d sit down. Rinse and repeat until he’d give up and walk off. If we really needed peace and quiet on the sofa we’d put him behind the gate in the kitchen.
Now he’s two and seems to have managed to grow out of it and can stay sensible when he’s on the sofa - mostly.
So really, I think Leo will grow out of it along with your management of not allowing him to rehearse the behaviour. He has a long history of mouthing and nipping, so it will be a long tail of the behaviour extinguishing, and there is always going to be one situation that’s last to resolve.