That is a tough situation. I agree to hold off on telling him for now, until you are established a have so trust/ safety. What comes to my mind is 'dignity'. Be yourself..stick to your own boundaries (or else you trigger your own complex emotions). Learn to trust him by how well he respects your boundaries). It is always your choice to tell or not...but if you choose to, you might want to wait until he sees and respects you as s person first. Then he can see you as a person who just happens to be treated for BPD.

And remember, you are not hiding it from him , you are exercising your own choice and power over wheb you tell people your deepest truths. That is self respect and self awareness. Good luck!