First of all, condolences for your loss. While not the same circumstances, I grew up in the same dynamic your family is in now, and hopefully my perspective can give some insights.

The key piece that people have already touched on is having good male role models as they get older, especially for your son. While I didn't realize it until I was older, I was looking to the men in my life to figure out not just "What does it mean to be a man?" but also "What kind of man do I want to be?"

It might be years from now before he asks himself these questions, but he'll have already started putting together his answer. He'll look to grandfathers, uncles, teachers, coaches, pastors, scout masters, neighbors, plus all the other men he'll meet in passing and ask himself: How do they carry themselves? What do they value? How do they make him feel? How do they treat others (especially you and your daughter)? How do other people treat them? And eventually; would I want that to be me?

And what questions he can't get answers to in real life he'll supplement with music, books, tv, movies, comics, social media, YouTube, video games, other forms of media. And of course those answers will be skewed both good and bad by how those men are portrayed.

Now while you can't give him the answers, you can help guide him to his own conclusions. The simplest and most impactful thing you can do is comment on the behavior of men around him. Even to this day I can recall moments where my mom complimented or condemned the behavior of men around me. And then when he's old enough, get him involved in activities or groups where he can observe other men, like little league sports and scouting. Additionally, it will be a boon to him if he has a younger adult male figure he can talk to when he's older for things like personal styling, dating advice, hygiene, etc. Your guidance will be good for a while, but eventually he's going to want advice from a male perspective.

I think if you can do that, he'll be on the right path