I feel for you so much as you face a number of challenges all at once: Someone else coming to live in the home, your son's change in behaviour and your wife's dismissal of your concerns. I suppose you could say it's a triple challenge.
With autism not always but often involving heightened sensitivity, I can't help but wonder what your son's sensitive to right now. Might take a bit of detective work in order to get to the bottom of things. As tranzcrybe suggests, could he be sensing and reacting to your change in behaviour? For example, if he loves his grandmother, does he consider your feelings towards her as some kind of threat to her? A highly sensitive person can often feel even the most subtle of feelings (such as subtle threat to someone else's happiness). Or could he simply be sensing your stress or annoyance and this is his way of managing that vibe because he doesn't know how else to manage feeling someone else's stress or annoyance? Is he someone who feels other people's emotions? I think this is where getting your wife on board is important. Kinda like 'I feel our son's experiencing some emotional challenge and we need to address it so that we can give him skills in managing this kind of thing for when it pops up again in his life, in the future'. Is there a need for skill development in the way of managing new and challenging emotions?
Perhaps the reason for your son's change in behaviour is coming from somewhere else. Could involve a couple of contributing factors. If this is his first year of school, could the change in his behaviour be coming from more than one disruption to his life? Could this be his response to stress? So much to wonder about. Something else that just came to mind: Could your son be overhearing how 'horrible' a man can be (with your MIL discussing her partner to her daughter, as an example)? Of course, not all men are horrible but does your son know that or is he being led to believe otherwise? Could there be a need for your MIL to not discuss her relationship with your son in earshot? In all fairness, maybe she's already careful in this way. Sorry about this; I think I'm leaving you with more questions than answers. Personally, I've found every good quest for answers can hold a lot of questions.
As a sensitive mum who raises 2 sensitive kids (18yo son and 21yo daughter), it's so much easier when you become a family of brainstormers, all putting your heads together in order to figure out what each person is struggling to get a sense of or feel for. 'Could you be feeling this...?' or 'Do you think you could be sensing that...?'. My husband, on the other hand, prefers not to become a part of the brainstorming team, as he's not a fan of emotional challenge. Do you have anyone in your life who's willing to brainstorm with you? Might pay to find a sensitive person who's able to get a good feel for what your son could be experiencing, so that you and your son can reform your relationship back into one with a strong bond. Perhaps it could even involve another parent who's a little further along the path of experience when it comes to raising someone with autism. Could this be a challenge that holds the potential to lead you and your son to develop a stronger bond than ever before? You become the man/father/friend/guide that helps guide him through perhaps the biggest challenge of his life so far.
