Thanks for your question and your loving concern for the needs of your child. This is a difficult situation. It's awkward because you and your ex have split up, gone your separate ways, and clearly moved on--you are with other people. There are others, though, that have not moved on: your son and the couple that he has always considered his grandparents. Based on their actions toward him, I would guess that they have always considered him their grandchild, as well, even though there was no blood involved.
Here's the thing: legally, technically--there is no reason for you guys to perpetuate the relationship. But don't we have those "honorary" aunts and uncles, those "second" mothers and grandmothers...who in the absence of the real thing or even alongside the real thing, are so very dear to our hearts? I have many relationships like this personally. My best friend's daughter is a young lady I have known and cherished as my own since she was around 7. She is now in her early 20s, and comes to me regularly for advice and maybe dinner or a movie.
The situation is compounded because as you say, your son had behavioral problems. These problems, especially at such a young age, might stem from or be connected to other things. In my experience as an educator, behavioral difficulties often walk hand in hand with emotional problems. Even at 6 years old, your son may simply not be able to deal with the emotional impact of the loss of this relationship, which is akin to his grandparents rejecting him or dying.
I would like to stress to you that you have done nothing wrong--you have honestly and sincerely ***** ***** do what you guys believe to be in the best interests of your boy, and that is truly admirable. My advice is to take a step back, though, and make a list of pros and cons of separating your son from your ex's parents. When you look at the advantages of doing so, are they favorable more to your son and his "mama and papa", or to your ex and yourself? Do they help you guys avoid the awkwardness of seeing each other, and having your new other halves having to deal with the situation? How does the separation benefit your son? Ask yourself the hard questions.
Afterwards, if you see more benefits in the separation, I would take a slower, gentler approach. Fewer visits, spaced out more. Fill your son's time, when he misses "mama and papa" with other fun pursuits and time with you and your new fiancé. Encourage your fiancé's parents to get to know him, and educate them on how to handle him so that behavior is not an issue. Perhaps he has sensitivities to noise, light, textures, etc. that make behavior difficult.
My best to you, and please let me know if I can be of further assistance. :)