The time out really should work but it does take time. Oscar stopped mouthing at about 4 to 4 and half months.

With the lead thing be calm but really firm. The chain lead might work but to be honest Oscar just moved from the lead to me when i tried that.

Try standing on the lead close to him so that he has just enough lead to lay down but not enough to chew or jump up and then ignore him. When he is calm treat him and try walking again (you will have to repeat this until he gets it).

Good luck x