This might not be what you want to hear, but you need to be clear and you need to accept that you will be the "bad guy" in this situation.
You feel bad about breaking his heart? Fine. But you already decided you're going to do that (and for good reasons!), so don't try to make your feeling bad about it his problem as well.
I'm not saying don't let him down easy, but telling him you really care about him and you want to stay friends is a cruel thing to do.
A lot of people don't seem to realize how manipulative this is (though to be fair, a lot of people probably don't even mean it to be, they just don't think about it and honestly believe they're being nice).
Because it's not making it easier on him, it's making it easier on you.
If he's smart and strong-willed, he'll flat out tell you no and take that breakup for what it is.
But since feelings are feelings, his thoughts might just go into this direction:
Fine, she broke up with me, but she did tell me she really cares for
me... there might still be hope.
It might even go as far as:
As long as I stay friends with her, I will still get chances to change
her mind. So it's not a total loss yet.
And then he does stay around you and you're constantly breaking his heart again and again (even if on much smaller scales).
Then you start dating and it gets even more difficult for him.
Only he can't even really blame you because you're just friends now after all. Obviously he must have known what he got himself into.
At some point, he might be able to get over you. But you're making it unnecessarily difficult.
It's kind of like saying
I don't want to be with you anymore, but I'm happy to torture you for a while.
I'm well aware that some people do manage to stay friends with their exes. As far as my personal experience goes (myself and friends), most of those friendships aren't real (meaning one side still has feelings for the other) and the few that are still had the person who got broken up with go through a tougher time than necessary before they finally moved on.
So if you really care about him: Be tough. Make a clean break and let him move on. Don't string him along in a friendship when you're fully aware he wants more.
You can try to befriend him again once he has moved on, though let's be completely honest: That might be never. (It might also be a few weeks after the breakup of course. Who knows?)
But do not try to befriend him while he's still hung up on you. I can't stress this enough. It might soften the blow at first and make you feel less bad about it all, but you're simply preventing him from moving on.
Or to use an analogy a friend of mine once told me: Just because you feel bad about shooting him doesn't mean you should feed him a slow and painful poison instead.