I don't know why your relationship with your son-in-law has soured; he may not even know himself. Our motives and emotions can be very complicated, and sometimes we do things without even understanding why we do them. This was the Apostle Paul's experience: "I do not understand what I do. For what I want to do I do not do" (Romans 7:15).
What does this have to do with your son-in-law? I'm not a psychologist, of course, but clearly something has happened in your relationship that now irritates him -- something that wasn't there before. I don't know what it is -- but (for example) could he be jealous of your close relationship with your daughter? Or could he feel that you're somehow trying to control him, or are questioning his judgment when you give them advice?
Only you can answer this, of course (and it may be something entirely different). But let your daughter know this situation distresses you, and that you want to move beyond it. If the only way to do that is to "back off" (to use your son-in-law's term), then it may be necessary, at least for a time.
Ask God to make you the best in-laws you possibly can be. Just as your son-in-law and daughter must adjust to their new role as husband and wife, so you and your husband must adjust to your new role as in-laws. This isn't always easy.