Speaking as girl who was hard to waken as a teen and young adult, I have to agree with the above. You're wasting your time, increasing your own frustration, and causing him to have sub-optimum sleep. Stop it!
You know the absolute last moment he can remain in bed--use it. If he absolutely has to be out of the house at 6:30, count backward. If he needs to shower and shave and eat, then he needs to be up at least 20 minutes prior. Set two different clocks with very loud alarms, one for 5:55am and the other for 6:05am and put them close enough to the bed to annoy him but not so close that he can just shut it off without getting up. That way, if he gets up and turns the first alarm off, he can still lay back down for 10 minutes if he decides to. If he lies down again after the 2nd alarm, turn on the lights and tell him to get up--once. No begging and threatening. You are merely doing him a favor by calling his name ONE time. Either he will get up or he won't, his decision. He may be acting like a child, but he's legally an adult.
Do not beg and plead with him. Use natural consequences. If HE chooses (that's the key word here) to stay in bed after 2 alarms and a verbal warning, you need to just let the chips fall where they may. He misses a test? Aw, too bad. Now he has a zero. His fault and his alone. He gets in-school suspension for being late again or missing too many days? Sucks to be him. He has the power to either get with the program or ignore it.
OP, sometimes as a parent we just have to sit on our hands and let them fail at a few things, suffer some consequences and learn life lessons. Your son is an 18 year old man. This is one of those things he is going to have to do for the rest of his life. You need to give over responsibility to him now. You are not doing him any favors with the game you and your DH are playing now
ETA: I learned my lesson in college. I could sleep through anything; fire alarms, loud clocks, loud conversations in my very room. All it took was missing enough classes that I nearly failed my 8am class to bring me around.
My DH learned his lesson at my hands.

He was a bit of a mama's boy and he expected me to do what his mama had always done. Hell to the no! I gave him fair warning that I would no longer be responsible for getting him ready for school (first year teaching.) One day I did my best to wake him up, to no available so I walked out and took myself to work. And he was late to school! It was the kick in the pants he needed. Having your very unhappy principal wake you up by phone apparently was the cure for morning sleepiness!