He's only 20 years old . He doesn't take pills or anything. Like we mess around he's fine but when we are about to it goes back down I don't get it

Does he use condoms? Does it always happen or has it happened only twice?

You do know the flagpole doesn't work like a light does. Nor as reliably.

Does he masturbate? If so, how often?

Two times out of how many? Did you make a big deal out of it, when it did ? Then second time can be worry from it happening first time.

Money, school, work, all the stress can make it happen, and every now and then, it just happens, Sorry, but it is not 100 percent, some guys have more failures than others at times, but that is just the way it works.

If it is happening all the time, then he needs to see his doctor for a check up