C.P.
Tell him the truth.
Tell him about summer-camp, pre-school and kindergarten. Take him there, if you can, and show him how cool they are. Let him know that he can't go to those places if he isn't going pee and poop in the toilet every time. You can give him the reality of the situation, even down to the fact that other children won't want to play with him and will make fun of him for not going in the toilet like a big boy.
I went through a short period of time with my youngest during which he refused to poop in the toilet. He'd only poop in his pants, no matter what he was wearing. My cure: I took his pants and underwear. No pullup, no NOTHING. He ran around naked until he agreed that he would only poop in the toilet, and he wasn't allowed in his room or any area that was carpeted (which left him with the foyer and the bathroom). It sounds extreme, but it worked. My boy was (and is) STUBBORN. It only took a few days before he caught on.
I told him "If you are just going to poop in your underwear and make a big mess, I won't give you your underwear. I don't like cleaning up your stinky poop. You aren't a baby. And, guess what? You can't go anywhere without pants and underwear. You can't sit at the table for meals, can't sit on the carpet and watch TV, can't go out and play....well...you can't do anything without pants and underwear except sit on the linoleum. You can't even come onto the carpet, because I don't want you to pee on it. You'll have to show us that you can use the toilet like everyone else in our family. You are the only one who doesn't."
If he agrees, proves that he can do it, you give him his pants/underwear back, and then he poops or pees in them, take them away. Keep them long enough for him to become quite unhappy about it, even if it's an hour or more. Make him feel uncomfortable. He SHOULD feel uncomfortable about peeing or pooping in inappropriate places. If he pees or poos on the floor, make him get the cleaning products, and let him see how upset you are about having to clean it up. Don't allow him to go off and play while you are cleaning it. Let him see your anger and disgust, and even give him a few stern, angry looks. Children want our approval, and as parents, we often forget the affect our disapproval can have in motivating them.
If he is successful, continue to praise the heck out of him. Brag about him on the phone (in front of him). Take him to buy school supplies and talk about how excited you are about him going off to school/camp. Bribery should not be necessary (and it doesn't work). He has to want to do it because that is what is required to be accepted in society.
I wish you the very best of luck!
C.