Three thoughts:
1. Have you asked him what special reward or celebration he would like for using the potty at home? He may have a much better idea than M&M's, and I agree that bribery is not a good precedent for getting his cooperation. You could make a suggestion or two and see how he responds. Perhaps try "We can send (day care teacher) a letter telling her you know how to use the potty at home!" Or, "Do you want us to sing your special song at home, too?"
2. When we're tempted to describe a child as strong willed, or our interactions as a fight, or any such antagonistic language, it's a good idea to look at our own ideas and attitudes. It's often true that we are providing the object of his resistance, which might well be our own strong-willed ideas, or the way we set up our expectations. My grandson, at 4.5, is still often resistant to going potty until he simply can't hold it any longer. But instead of fighting to get him to go, we either let him decide (and honor him with our trust that he will), or make the experience fun, as in "Hey, G, there's a bathroom full of dinosaurs, and they said you can't come in. What are we gonna do about that?" G, of course, wants to get right on it.
3. If you can bring yourself to try this, tell your little guy that you are so happy he's big enough to know when to use the potty. Let him know that it is completely up to him, and you trust him to notice how much easier it is to use the potty than to have to get cleaned up after an accident. He may need to test your word for a few days.
Or he may not be quite ready yet – school plus home is more than twice the commitment. K. often train, then regress, then train again a few weeks or months later, when they are truly prepared to take on this rather large commitment. It's a huge step forward, and it means accepting that they'll use the potty every single time. He will want to do it when he is truly ready. That sometimes happens a little later for boys.