Sounds like he's cottoned onto the fact that he can play up in the ring without fear of correction.
Try making a full metre of space away from him on the lead so it's harder for him to have a go.
I'd also consider having a ring made with a bit of a prick on the outside, wear it so when he tries to bite at your hand he smacks himself and gets a prick.
In the end though you may just have to discipline him soundly in the ring and be done with it.
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