He refuses any offer to play a sport or join any activity other than a sleepover. I don't know how his friends put up with his constant banter and demeaning tone. This has been going on for a while now and we are out of ideas. His mother and I believe we have been very caring and patient with him since birth but we must be doing something very wrong to get this result. If nothing changes I fear he is headed in a very dark direction.
First of all, you are not alone! You aren't the first parent of a preteen with a toxic attitude. I know that doesn't solve your issue, but I want you to understand in most cases, it isn't what you are doing wrong. It most likely points to the fact that something is wrong. And that's truly where we need to focus. The first and most important step here is determining what is causing the negativity.
We can't develop a plan or solution until you have a sense of what the root cause is. As a parent, you are best equipped with the knowledge and history of your son to be able to point to the next layer of understanding here. Is there a particular time or event that you can point back to as to when the toxicity or negativity began? Outlining a timeline can be helpful. Was there a life event or something that happened as a family, etc.? If so, you can begin to peel back the layers of feelings around that particular event. Some of these events can be more obvious than others. A divorce or death in the immediate family, a serious illness of a sibling, a job change that requires switching schools, moving, bullying, friendship troubles, etc. These are all examples of a situation that could serve as a trigger for a major change in attitude. One of the things we're getting at here is if there has been a change OR if the attitude is characteristic of his temperament. Some kids generally have more of a flat affect than others.
If you can't identify a trigger, then you can start to explore social and/or psychological factors. Is this attitude typical of his peer group? Is it typical of your family life? Has he recently found a new friend group that you aren't as happy about? Perhaps you want to enlist the support of someone from his school community. Maybe a counselor or teacher has a concern or thought that would help put some pieces together.
Lastly, I would rule out depression. There is a long-term, low-grade type of depression called dysthymia that often presents itself as a chronic negative outlook on life, lack of interest in activities, and low energy. It would be worthwhile to have a licensed psychologist or medical professional conduct a depression screener.
It's not easy guiding our kids through adolescence. But you aren't alone. I strongly encourage you to build a network of support for both you and your wife as well as your son. You are on the right path just thinking about this and hoping for some answers. Keep it up!