💑 Remarried after being widowed, and my kids are struggling to accept my new husband

My first husband passed away almost four years ago now, and I remarried last year to a genuinely kind man who has been patient and understanding throughout this entire transition for our whole family. My two kids, thirteen and ten, were told well in advance and seemed generally okay with the idea in the actual months leading up to the wedding itself, but since we actually moved in together as one household, my older child specifically has become noticeably distant and occasionally openly resentful toward him. My younger child has adjusted more easily, actually seems to genuinely like him in most everyday moments, which honestly makes the contrast with my older child's specific reaction feel even sharper and more confusing to navigate day to day. Talked to my older child directly about it, and the response was mostly that he is not dad, said flatly, which I completely understand as a genuine feeling, but that leaves me genuinely unsure how to actually respond constructively to it in a way that helps rather than pushes them further away. My husband himself has been genuinely careful not to overstep, letting me handle most direct discipline and not trying to actively position himself as any kind of replacement father figure, which I appreciate given the situation, but I also wonder if that careful distance itself is unintentionally reinforcing my older child's specific sense that he is somehow separate or lesser rather than actually part of our family now. A grief counselor we saw briefly after my first husband passed suggested this specific reaction is fairly common among older children specifically, since they remember their biological father more clearly and vividly than a younger sibling typically would at a similar age when the loss actually happened. I do not want to force any kind of forced affection or rush this timeline unnaturally, but I also genuinely do not want my older child's specific resentment to define our whole household dynamic going forward for years to come. Has anyone navigated remarriage after loss with kids reacting this differently from each other, and did the more resistant child eventually come around on their own timeline, or did it require something more deliberate on your part to actually help that happen?