Iím sorry that you and your son are not in best relationship but I do hope that both of you will find way to each other. It is much healthier to be close to parent/child than other way.
I have some thoughts related to your case and I will write it; but please bare in mind that Iím trying to give shot into other direction in finding some answer; this is simple guess. So Iíll try to hypothetically find possible explanation of your sonís behavior but excluding sexual abuse as cause. This is my vision of one possible explanation of your situation; forgive me if I was totally wrong, in that case consider this as some brain exercise
It is normal for child to fight with parent trying to find his own place under blue sky.
But it is very hard to exclude on purpose someone to whom bond is very strong (bond mother-child). This is something that should have significant root, something that is difficult to ignore or miss. Here for me is difficult to think on possible sexual abuse as source of your sonís behavior; his act was more act of fighting than act of objecting for lack of attention.
When one person (child) tries to exclude other (parent) from his life by cutting relationship with him this could be explained like desperate move. So desperate that child hadnít had any power or other mean to force his way out of particular situation. He was so desperate that couldnít think on any other option in resolving conflict or whatsoever. This was struggle for bare survival.
Doing this your son/child admitted that he was badly hurt; he couldnít fight anymore; his all defense has had crumbled (please try to perceive his boundaries talk in his letter to you like complete lost of his defense) so he needed to completely exclude himself; it was done mainly for hisown protective purposes. By doing this he has also tried to punish you and somehow to bring you in order (this is main message from inferior person to other stronger person/side in this kind of ďfightĒ). Heís lost all other weapons and this was available option. At the same time by cutting bond with you heís hurt himself too. Child is more sensitive than parent by its nature; child needs love and care for wellbeing and consequently child has been more hurt by this action. At the end this acting was destructive for both sides. So destructive that damage is almost irreparable.
What could be source of all this? Please try to answer to yourself how was your childhood, where you happy child, where your parents supportive, loving and carrying? Have you deep connection with both of your parents now? If you had some unresolved issues for long time with one or both of your parents this could be at least part of answer. On other side your sonís first 3 years were most important for his emotional development, and if you had intensive fights with your husband at that time this could give us some more light on complete picture. Usually when mother is endangered by partner she can turn all her attention to child and by this mother can develop overprotective relationship with child. This is surrounding which is not healthy for small child who is developing his emotional capacity. Overprotective parent knows always what is best for his child. This kind of parent has a lot ambitious plans for child and can easily not be aware of real childís needs and feelings. Such relationship can give many scars to child and at the end child is brought to huge pressure and needed to fight his way out; this fight actually might happen many years later and this can lead to separation.
Iíve read book Toxic parents by Susan Forward, please try to find it and read it. This book explains way for child to recover from such parent-child relationship and gives full access to its psychology.
One of advices to children in this book is to write letter to parents. This letter is very special one; it has been written in advance with some preparation (there mustnít be any hurry), in steady, calm and secure atmosphere. That shouldnít be phone call, talk or whatsoever but letter with written words. Its message should be as clear as possible; aim is to avoid possible misunderstandings. In this letter child should tell all those things he/she objects to parents.
Iím not saying that your son has written you letter in this exact way and that you are bad parent. I presume that heís chosen to write you because he wanted to be understood very clearly. For your son this was very important...
Message with boundaries in his letter worries me; by this heís stated that you used to cross his boundaries and he warns you that he wonít take it anymore. Those are heavy scars and if Iím you I would be alarmed here. Please think more on this and try to read his letter again; maybe there are other similar messages?
For both your sake please try to be as supportive as possible. Have you written him letter back? You can send him letter with general supportive attitude, it doesnít have to be long. Give him message that you will try to be as cooperative as possible and that you miss him. Please try to act like that by accepting his terms and conditions in approaching you back. Iím sure that his approach to you is related to recovery of yours relationship; this is healing, try to feel it. Booth of you needs this and deserves it.
I wish you best!