Force him to sign a contract with you and at least one other witness to watch.
Find a good contract describing the rules, regulations, and requirements for your new relationship and elaborate it to include all the things you’d like. Make sure it gives you the official and undisputable rights to train, humiliate, tease, cuckold, punish, discipline and chastise him.
You can put him to task with coming up with a strong contract and sending you the draft. Be careful to never accept the first draft he sends you… instead add in lots of more rules and make the conditions much harder and only in your favor. You can send him back the contracts and have him review and modify it, but be sure he accepts all your requirements and changes.
You may want to enlist a friend who can act as your “lawyer” for the contract. Have him send drafts to her, and have her revise the contract in your favor. Tell him that your lawyer will decide on what you can sign, so it will leave him completely at the mercy of the other woman.
When you are ready to sign it, have your girlfriend (“lawyer) over to witness the signing and take a copy for safe keeping.
Toys, Discipline, Panties, Dresses, Diapers, and such cost money….Upon signing the contract, make him deposit a reasonable amount of money with you for his training.