The truth is the only way to go. It has already taken you long enough to come clean-- don't let it go any longer. He may forgive you now and honestly a lot of men don't seem to have an issue dating older women. The issue you have created for yourself is that you lied. So, tell him the truth and be sincerely apologetic because it is a big lie. If you keep letting him believe that you are younger than you really are he will eventually find out the truth especially if you do move in together and it will be a MUCH bigger deal.