I agree with Julie, in person is always the best. That way, as she said, you can "read" his reaction, but also, so that he can see that you are sincere when you apologize for not telling him before you had intimate relations with him. One suggestion that I have would be to have, in hand, information about this virus for him to read and possibly answer any questions that he may have. And yes, be prepared for him to be mad, hurt, upset, whatever, but hopefully after you provide him with the facts it will ease his mind enough for ya'll to have a conversation and work through this.
Good luck and big hugs,