Family court place children's rights above all adult happening between themselves. The child has a right to see and spend time with both parents. As there are no court orders regarding your 10-year-old son, he does have a choice and can come and stay or live with you if he wishes, provided you can care for him on a daily basis whilst he is there.
At age 10, a child must not be left at home without adult supervision. No parent has the right to stop another parent from having contact or access to their child unless a court order states this. As you and your ex-wife do not have any custody orders of your son, your son can ring you, see you, stay with you, visit you as he wishes.
He can meet up with you away from the mother's residence and suggestions such as picking him up from school, a friends place or having a friend or family member pick him up and return him or other such arrangements can be organised.
Should it be that your ex wishes to keep you away from her place of residence and have no contact with you then this may be a solution or communication between you both via email may be possible to come to some arrangement.
I suggest speaking to you son and asking him first what he would like to do regarding contact with you via phone, visits, staying over, living with you, etc., equally so with regards to his mother and to listen to the child's reasons. Your son may want to live with you as he misses you but also living with his siblings as a family unit. Most young children wish their family to be together and do not understand the dynamics at play but simply wish to be a family unit again. They want to freely see all family members and spend time with all. Or he may not like seeing his mother with another man who isn't his father with no understanding.
Your older children would be more vocal, would articulate their distaste of change in the family dynamics nor accept that their parent has a new person and a new relationship always changes a person.
You are free to file for custody of your son, however, you will need to attend mediation which will include involving your 10-year-old son, as they will take into account his viewpoint. One thing parents must consider is not using their child as a means to gain access to their ex or use children to gain some kind of control or revenge or communicating messenger between them which is extremely common.
A lot of family members, friends and especially children including exes, find it difficult to adapt to 'change'. Your ex may very well be behaving differently as she, just as you are, is currently adjusting to change as well. 25 years of marriage plus more of being with each other is a very long time and a huge adjustment for everyone involved including those outside of immediate family.
6 months is not much time for any dust to settle from this major upheaval in your relationship and should you gain a partner. You, too, will change as different people bring out different sides to us all. But to the children, the Dad will always be the Dad. The Mum would always be the Mum and we all know regardless of children's ages whether 10 or 50 years of age, children don't like seeing either parent with a different person and it takes some time getting used to it, if they ever do.
However, if your ex-wife will not communicate with you, she will have to communicate via email when it comes to your son and will have to take into account that he does have rights to see you and have as much contact with you as he wishes. However, as adults and parents, we also have to try and help all children regardless whether they are a child, adolescent or early 20's, to adjust n deal with a major living and lifestyle upheaval and disruption to a family unit.
Even though both partners are also handling equally the difficulties of change and beginning a new lifestyle adjustment of they're now, so it would be interesting to know whether your ex is behaving differently because she herself is adjusting and has met someone which everyone is not used to and they are seeing a new side of her that they haven't seen before or she is experiencing freedom and forgetting her duties as a mother to her 10 year old son?
Either way, you are not breaking any laws and your son has every right to have access to you as much as he wants. Corresponding via email regarding your son is not only a way of communicating to your ex regarding your son but also can be used as evidence that you have tried to communicate on an adult level to sort out access problems to you son and her replies will show insight in court whether she is putting her son first before her issues if any.
One thing for sure is the courts will not allow children to be used, pressurised or be pulled and push to into choosing sides, choosing between parents or alienating one parent from the other as both parents are equally important in the development of children.
Good luck and I hope all goes smoothly for your son in feeling that he is loved and free to see and spend equal time with both parents.