How can I help my 21-year-old son and his 80-year-old grandparents get along?

I seriously doubt you can help. I’ve seen this before and sometimes the generational gap is just too wide to bridge.

I think you should remind him that its their house so its their rules. He is a guest and should act accordingly. Grin and bear it till he can move out. That is all a part of being an adult.
He should also make an attempt to communicate with them without you being the go between. He has to do the adult thing and learn to resolve issues on his own.
My mom has the same issue with my neice. I explained to her that her granddaughter walks away because she doesn’t want to make her feel uncomfortable in her own home. And I told my mom to teach her to cook since that is something they both can share doing together. My mom would do the same thing to my neice. She would leave the room when she would enter because she felt my neice felt uncomfortable. Now they both are working things out and communicating more because they share an interest. You son should find out what they like to do and do it with them so they don’t feel like strangers.

I agree with @Pandora. It is their house so it really does fall to him to make the real effort to fit in with their expectations, even if those expectations aren’t always so reasonable to a young man.

Can he find a share house with people of his own age? Just to help resolve the potential for ongoing conflict that is going to be stressful for them all?

@Bellatrix : Yes, a room in a house with people near his own age is a long-term goal.

@Pandora : I believe asking my dad to teach my son some things about cars is a good idea, and I think my son would be receptive.

Tell him to suck it up and be nice to them because in a few years they will be dead and he will have to live with himself.

cars might be a good bond….Good idea!!

First, you need to not take this on emotionally. You are not there, physically, and you are not a direct party so you can’t “fix” it.

You can talk to your son about trying to see the world through your parents eyes. Because they are in their 80s and are unlikely to be able to change.

What are the specific issues? Knowing those might help us suggest fixes.

Do your parents articulate specifically why they feel disrespected. Maybe if you give your son very specific things to do or not do, the conflict will lighten up. He cannot read their minds, and he might feel like he is walking on egg shells.

Also, remind him it is just for a couple of months, and then he can move out into his own place. While their he should show them respect in their home. But, I would not lecture him about respect in their home as the starting point, I would start with asking him to do the few small things that might be bothering your parents. They might be making him feel like a bad kid, or trying to control his every move, their exectations might be too high? Obviously, I have no idea what the specific problems are.