I know you don't want to hear "it's a stage", because for Mother's who have gone through this we nod their head saying "ahhh poor girl, this too shall pass", Mother's who are actually going through it at this moment are saying "AGGG, what do I do before I pull my hair out"
Here's what you do. Make it a game. Sit him in a room, say "Bye bye, Mama will be back" and leave the room, 2 seconds later, pop back in, "YEAH Mama's home" and have a little dance, a hug and a kiss and then leave the room again, this time wait 5 seconds and so on. He will think is this a game and all of a sudden "bye bye" is NOW fun (because right now all he knows is that "bye bye" sucks)
Do it a few times a day and gradually you'll see that you can stay out of the room longer. He will start to get bored waiting on you to return and start to entertain himself, but he knows you'll come back and when you do, it's FUN DANCE PARTY TIME!
Mark my words, when he leaves you running into to Pre-K, you're crying and he's saying, "BYE BYE see ya Mommy", you'll remember back to this time and miss the old days for just one moment, then you'll smile because you raised a well adjusted child.
Good Luck, God Bless!