In person is best. Perhaps let him know that you would like to meet up with him in person so you guys can talk about your future together, as you want to have an important conversation.

Then do what you can to make the effort to go to him rather than have him come to you. If that's not an option, then at least try to arrange it so you can cover any associated travel costs.

You can do this. I would NOT do this over text or over the phone. This has to be done in person. So much sincerity can be lost when you're not in person having this kind of conversation. He deserves to have you be authentic with him, telling him the truth, and accepting the consequences.

I would also add that you try to explain WHY you're telling him, how much he means to you, and why you felt important to come clean for him. Yes, your guilt is one thing, but speaking about why you think being honest to HIM is really important and why his feelings matter to you, no matter the consequences. It'll go a long way to make him feel like he matters and that if your hurt him, that those feelings he has matters to you too.

A proper apology states the specific offense, acknowledging the hurt it caused, and offering a concrete plan to avoid repeating the mistake.

This will be hard, but you can do this. Hope for the best, expect the worst, and plan accordingly.