I have known this guy for coming up 2 years and he has been one of my closest friends from the moment we met but he is 3 years older than me and my parents are unaware we recently started dating they have never met him and are unsure telling me that I should back away from him.he is an amazing guy and I want my parents to except.our relationship.

How would I go about making them see that he isn't just another guy?

Dating alone is fine, sexual relations is where the trouble starts.
It all depends on your location.

One of the biggest leaps into maturity is when you realize that no one 'makes' anyone see or realize or accept or like anything or anyone.
Parents make children do all sorts of things, but even they don't 'make' them feel a certain way.
This may not seem important, but it's huge.
You can work on many ways to show that you are a responsible 16 year old, from homework and grades to chores and HONESTY, and HOPE they accept him. You hid this friendship and the recent dating, and I don't see how you expect to suddenly introduce him out of the blue. So you are going to have to be very gradual about him, and say you've known him for 2 years and like him, without saying ONE WORD about wanting them to accept him!

What is a 19 year old doing hanging out with kids still in school? What is he doing with his life? Job, college? I hope he isn't just hanging out, trying to avoid adulthood.

How would I go about making them see that he isn't just another guy?

By being upfront and honest about seeing him, and letting them meet him.

The first thing you need to do is be honest with them. It will be hard to accept him knowing you've lied about dating him. Come clean with your parents.
I'm wondering why he is dating a high school student. What does he do while you're in school?

So you go to your parents and tell them you have known this guy for 2 years as a friend. You and he want to date (While I don't advocate lying to your parents, I would not mention that you are already dating). Ask if this is OK and offer to have him come over for dinner so they can get to know him.

The age difference (19-16) is, in my opinion, on the edge of OK. Personally I would suggest waiting until you are 18.

Do NOT continue dating him until your parents are on board.