One way to explain this to him is to ask him to consider that you think it is committing too soon and too quickly which led to your problems in your marriage. That is how I read this situation. Therefore, making such a great leap so soon is more likely to cause you to yo yo than reduce the effect.

It is too early for a collar, certainly a 'proper one' where you promise complete and long term service. Maybe you can compromise down to a training collar? Maybe this is what he means? I would ask him what he thinks a collar means in this context, talk about your feelings, negotiate something which allows you both to have your wish.