I think if you care about the man at all, you should tell him now. It would be deceitful to keep something that heavy a secret from him. I agree that he will be hurt and that you may also lose him. If you continue your relationship after concealing it, it would be a relationship built on a huge lie and it will certainly haunt you forever.

This is between both of you. Tell him. Hopefully, care and understanding and 2 people working together will come to the right solution. Good luck!