I am with a widower who lost his wife to MND. My situation is different to yours somewhat as our children were adults when we met (and when his wife died). He and his wife had also agreed to split just before she was diagnosed, all amicable with no other parties involved or on the scene. They had simply grown apart but still had the upmost respect for each other. He stayed and looked after her. Had he not and continued instead with the plan to move out it would have been a massive red flag to me. I knew him before she died but nothing happened whatsoever until 9 months after she passed and even then, we moved very gently. So much so that it was questionable in the early days if we were even dating.
We were and remain clear that our children, even though adults, come first. I am divorced rather than a widow, but have seen first had what it looks like to your child when their father deprioritises them for someone else. In the end they sat us down and told us we need not be so cautious on their behalf and have been nothing but encouraging in our relationship. But as I say his sons are adults with their own lives (one recently married) and do not feel they are competing with me for their father's attention. I can see how things might be different if there are younger children but I think the issues typically occur in cases where the integrity of their late mother's place in their family is not sufficiently respected or upheld.
I think it is perfectly possible that a person has done some of their grieving before their partner dies if there was a long illness. That said, there is still more grieving to do once they pass. I would say it is critical that you talk to him about his grief and how he feels. My partner's wife was a massive part of his life (they were together 30 years) and that would have been the case even if she hadn't died. We still do talk about her sometimes, as do his sons. I don't feel concerned that she remains part of their lives in this way; how could she not? I would be concerned if she weren't mentioned. I certainly don't think it is necessary the case that if you are with a widower you are second fiddle, same as you won't necessarily be if you are with someone divorced. I do not feel second fiddle at all, in fact quite the opposite.
Love bombing is a modern concept, is it what we used to call infatuation? If you meet someone you truly connect with then it is easy to go all giddy and things become openly intense quickly. That is how a lot of relationships start and things either fizzle out or it morphs into something long term. I did not openly show how I felt and nor did my partner. Perhaps because we are older but also because I think it scared us both. But we knew anyway. It is how you feel that is most important. Again, I think you should discuss this, including your concern that you might just be another in a long line of recent partners rather than someone special. It is perfectly plausible you are special.
Ultimately, if you are considering anything long term my advice is that you talk to him. Only then will you know if he is ready to move on and how to navigate things. For me, the red flag would be a refusal to talk about his ex wife and a desire to act as if she was never there.