You could always write her a heartfelt letter. That way she can have time to soak it in and react in whatever way comes natural without potentially offending you (if you were to tell her in person). That way you can always say everything you want to say with a lot of thought. If I were to have a hard time with something that would likely make me feel a strong initial emotional reaction, I would want to be able to react privately;-)
I totally agree that your first priority should be your son. After your husband died (((hugs))), your priority and focus in life falls to caring for your child and should always be your #1 priority. So, I totally agree with you that you have it right.
Just one thing to consider, though (and I'm speaking with experience from the child's point of view, though I was a teen, which I'm sure makes a difference). In a marriage, if you want it to be successful, you're spouse can't come second to your son. That is a set up for failure. Your new husband will not appreciate feeling like the second one where his feelings are always put after the son's feelings. (imagine if your new husband always put his daughter before you and your feelings, would that bother you?) But your new husband can't come before the child either because that's a set up for failure with your child since your child IS your priority and SHOULD be.
My mother got remarried after my dad left, and it was an awful experience. He started off seeming like such a wonderful guy, but he turned into an "evil step father" and it was hell. It was horrible. My mom put him first, and the rest of us second...BIG mistake. But her marriage wouldn't have lasted as long as it did had she put him second. So, either way, she seemed set up for failure.
My experience might be extreme (there's a lot more details), but from what I've seen, there are so many other kids who have felt the same from remarriages. It does not go as well as it seems like it will in your head. Remarriages usually are for the parent, not the children, though the parent THINKS it's for the child too. My mom got remarried, thinking she was getting us a new dad...but that's now how it works. My dad is my dad and will always be my dad. The man she married was HER new husband not MY dad.
Another big thing to consider is that he's been divorced. That REALLY does add a new element to it. The odds are against making that work. There are other posts in here from women who marry and become step moms and it's an awful situation. My mom ended up getting divorced (is currently getting divorced). It makes me upset on many levels...one big one being that as kids we went through all of that for pretty much nothing. Maybe find out what caused his divorce and see if you feel confident he'll work hard through any hard times you'll have.
My point in saying all this actually isn't to say you shouldn't get remarried. For me, based off of my experience as a teen, I have swore to myself I will never get remarried. I just can't do it. But every person and situation is different. The reason I'm mentioning all of this is to impress how different getting remarried is when you have a child that is not his and when he has a child that is not yours. There are many factors that come into play and it's a much more complicated situation. It's not the same as marrying a man with no children involved or a child from both of you involved.
Hormones and love and desire will make you want to think that it'll work out and be really good...and maybe it will be. Maybe the fact that your son is much younger (how old is he?) will make it more doable. My youngest sister did much better with a new father than us teens did. She really considers her step dad to be her dad. She was only six when my parents remarried. But he also was actually nice to her and cared about her. Not the case with us. He was very emotionally abusive to the rest of us. He's one of my least favorite people on this planet. (and he left my mom for a girl he met online! he's 57 and she just turned 18. he started the relationship when she was 17! gross)
Anyway, I don't mean to try to sound all negative. I definitely feel a concern when I hear people choosing to get remarried. Mainly I worry about the children. And I worry about how a marriage will work if the new spouse is not put as the priority...but I believe the child should stay the priority. So it almost seems a set up for failure. I know they aren't always failures, though! Maybe my experience just gave me a really bad taste in my mouth about it for me.
Sorry if I gave you more advice than you wanted! Ignore it all and just read the first paragraph if that is all you were really interested in. I wish you so much happiness in your life. You've been through so much already. (((hugs)))