If he JUST lost his wife after giving birth to his child, I wouldn't just "take it slow". I would give him at least a year, year and a half to grieve and heal without getting into any kind of romantic involvement with him at all.
He may SEEM like he's ready to pursue a new relationship, even slowly, but I assure you he's not. If you want to give this potential relationship a fighting chance, don't start it. You want him to take it slow, but he's not going to. He's looking for someone to immediately step into his wife's position. You're not even divorced.
Listen. You both liked one another in high school but it's not high school anymore. It may seem like a good idea to see where things could have gone when you were kids, but things have changed so much since then and neither of you are in the right place to be starting a relationship. Especially him. There is no way he's going to be able to start a slow, healthy relationship with you at this point in time.
You want him to keep it slow?
I totally agree with crab.
He's not available for a relationship. Not even remotely. I honestly think that if you really want to be there for him, you should be his friend and not even think about romantic involvement. I know you don't want him to think you don't want to pursue something "in the future" but that's something you should revisit when "the future" comes, not now, two weeks after his wife's death.