My suggestion...just tell them straight out. This is what I do with my parents if I'm going out with a guy. If you keep this bottled up for a long time, it's going to get you stressed out and then sooner or later something is going to slip out. Telling your parents about him can be nerve racking because whenever I tell my dad and mom about a guy they want to know EVERYTHING! However, I know they are just concerned for me and want what is best for me. I don't know your parents, but this Marc guy sounds really great and I'm sure your parents will like him too! What you can do is bring it up casually and once you tell them everything about him, ask them if it is okay that you bring him over to have dinner with your family.
Telling them lets them know that they can trust you. If you don't tell them, what's going to happen when they find out through someone else that your dating Marc. They are going to feel like they can't trust you. So you're better off telling them instead of hiding it forever.
My advice may sound kind of stupid but practice to yourself what you are going to say to them. If you practice ahead of time, it will come out a lot smoother so you don't feel so anxious to get everything out at once. Your parents sound like they really care about you and love you very much, and if they do, they will listen to you and trust that you will make the right decisions. I know it's going to be hard, but trust me, as soon as you tell them, you will feel a lot better!
Good Luck and I hope I helped.