Since he is your husbands son, i take it that you are not the mother.
Could it be that your husband harbors bad feelings about whatever happened between the mother of his son, his son and him or that he felt, he couldn't be there as a father back then?Could it be that they didn't went through the process of the adult child emancipating itself from the parents, which always is difficult?You have described their relationship as "more like best friends". Is your husband having good friends and social contacts aside from you and his son?Also being best friends doesn't necessarily work well with living together. I have seen some close friendships die out as living together removed the aspect of choice from them.
I think in addressing the situation it is important to find out what your husband sees in spending so much time with his son, but also how the son feels. From your description it does not sound like their current relation is a "healthy" father-son relationship for two adults. Of course it is good if the relationship between adult parents and children become more like friendships, but they are not the same and they should not fill that role in either sides life.
I think if you can carefully disentangle some of these aspects in talking with your husband, you could help them move on in a way that is sustainable.
Children and parents shouldn't be "best friends". When the children are adults, and especially after having moved out of the house, they have their own social life, their own partners, their own friends, they should stand on their own feet economically and with their households.
Of course i can understand the moving back in temporarily and needing some "familiarity" in the form of family after the break-up. However that cannot go on indefinitely as it risks falling back into being too dependent and recreating relationship patterns that befit parents and non-adult children, but not adults.
I might be a bit biased though as i moved out at the youngest age of all my siblings and i had an older sibling stay very long with my parents. Towards the end of that everyone was just getting on each others nerves but seemed unable to break out of that relationship pattern. For my parents it was the last child to move out, so they felt scared of having to deal with the fact that this phase of their live is now over and a new phase begins. In the same wake for that sibling it kept them back from finally standing on their own feet, making their own decisions and finding their own solutions. After moving out at last, everybody involved got much better for themselves.
Spending time with your parents, or adult children can be great and in our time probably happens way to little. But if it is the center of each others life, that is not good for adults imo. And well, as parent or child the rights and duties you have upon each other are different from the rights and duties you have with your best friend. Your best friend most likely didn't wipe your butt when you were little. Your best friend is most likely not expected to do so, when you grow too old to do it yourself. Your best friend can give you great life advise, but it isn't their responsibility to steer you towards choosing a solid path in life. And best friends should not come with the baggage of having made mistakes, that parents inadvertently make in doing so.
Finally in regards to old patterns, when i moved out, my parents "got off my back" at last. It took the physical separation to really acknowledge that i am a self responsible person now and vice versa for me not to lean into the inclination to have my parents help me sort my things out, when they are within my ability. So in regards to OPs question i see the risk of them living out a time they had, or wished that they had, when the son was just a boy. But that time is not coming back. Trying to hang onto it, will cause more harm in the long run.
As a bad metaphor: No matter how much you loved that one hoodie you had as a teenager, eventually you grew out of it. There is no fitting you back into it, no matter how hard you try. You would only make yourself suffer as it constricts your body and eventually it would tear. It is better to cherish the memory of that hoodie, but wear things that fit you now.