I guess in your shoes, I would go with my wife to a joint counseling session with a therapist, to talk about what you expect in your marriage. It does not seem unreasonable to request that you two can be together without the boy sometimes, no matter what his illness is. You need to be able to say this to her in a way that she really hears. Sometimes saying it in front of a counselor who can help moderate the discussion works better than just saying it at home (which of course you have been trying and it has not been working). I don't think having a child sleep with the parent is a cure for OCD, and the fact that she feels obliged to be in the bed with him even on the occasions when he has fallen asleep alone makes me feel like she is in need of some help in separating from him, whether he has any issues or not. But it would be better for the counselor to say this to her rather than you, so talk to the counselor together.
I agree with Annie. This doesn't sound like it's totally driven by the child, but she is adding to it by getting something out of this very needy child.
An aside, I have found clear links between erlichia and OCD on the internet. They are posted on other forums, so I can't post them here, but you can find them too if you good the two terms. Since most cases of erlichia are in dogs, not humans, most of the articles I saw referenced dog OCD behavior.
Anxiety tends to be an inherited disorder - is there any history on either side of the biological parents? Often, an "issue" will exacerbate anxiety and people go on a "hunt" to find the "cause" - probably the anxiety seed (OCD) was always there; just waiting to bloom and grow.
Is this child seeing a medical mental health specialist with experience in anxiety issues as as child psychiatrist or child neurologist? The best approach to learning how to manage anxiety is a multi-modal approach where therapy is only one component. OCD is an especially stubborn anxiety disorder and I would suggest you find help as soon as possible. Anxiety is an issue where one does not outgrow it nor does it go away, but a person can learn how to effectively manage anxieties/fears (this is what your stepson will need to do - with help, of course).
As to whether your stepson cannot stand the thought of you being alone with his mother - our child who suffers anxiety was too scared to be alone - a very common feeling of those suffering from anxiety. Our child began treatment before she was six years of age and our child psychiatrist reprimanded us for starting so late (the earlier the diagnosis and treatment, the better the prognosis). And, frankly, he is quite old to be starting the process of "managing his fears" (but there is a "never too old") I wish you the best ...