The trick to confidence in the show ring is to attend as many shows as you can and if you get there early enough, take a few circuits around a ring before the show starts.

You need to get him used to the environment and the atmosphere of a show.

Once he becomes an "old hand" his confidence in the ring should improve.

You are unfortunate that you have started late in his life and now can only show in the Junior or Open classes as it is in the "puppy" classes that dogs usually get this experience when there is no pressure to behave well!

If you can find a "Ring craft" training course near you, that could help as he will gain experience in "showing" with strange dogs of all shapes and sizes around him.