How to tell my parents?

Well, since you'd technically be an adult, and he would be one as well, there shouldn't be a legal problem. Like Silent Rain said, if your mom was okay with 20, I don't think 2 more years would be a problem. The only thing I would say other than that, is to simply approach her with caution, and respect. Apologize about not telling her the truth earlier, and try not to argue about anything, simply tell her that you kept it from her, and what his actual age is. Good luck!

If your mother is okay with him being 20, then I'm quite sure she'd also be okay with him being 21 and eventually 22. The difference really isn't so big. When I had your age, I used to be with guys who were up to ten years older. 4 years isn't a big deal and I'm quite sure your mother will be okay with it. If your parents love him and it's been going well, then you really shouldn't worry that he's nearly 22 and not 20.

Around where I live, we have this rule, half your age plus 7 is the lowest age you should date, and in doing the math for this guys age, the relationship seems to be of respectable parameters. :P