Strained relationship with son.

Edge, there has been some good advice given. I would say though that if you tell him he has been rude, or thoughtless or any other accusation it will cause more harm than good. He will feel you don't respect him as an adult. Correcting his behavior is now his spouse's job. He needs to know how you feel though. That you feel insulted and taken for granted. When he chastises you for something like not answering a text, it does no good to apologize or give reasons.
Instead, complement him on his ability to see other's mistakes and say you will try to do better. Make it your fault (even though it isn't) and he might respond better. As for having time to yourself, just say you lack the strength to see others everyday. If he insist, hold your ground: "No, I just am not able to do that." Adding a "sorry" is counterproductive.

Too often, AC do take their parents for granted and fail to give them the same level of respect they would a total stranger. The AC doesn't recognize that their words and actions are causing their parent emotional distress and hardship. They see us with time on our hands and a desire to help them out, just as they always have. I think we are all hardwired this way. Help you AC know what you are feeling by showing some vulnerability. Point out to him that being called things like "wicked" hurts so much that you could never have anything to do with any other person that caused you so much pain - so its a good thing you love him, but please stop hurting you.

I also see parents treated like door mats when it comes to their AC's demands. Boundaries work both ways. Set some and stick to them. He'll either learn to accept them, or the relationship will continue to deteriorate. There is only so much you can do.

And some grandparents jump through too many hoops to be able to keep seeing the grandchildren. I get that to an extent. But it’s an abusive son? No way. He thinks he holds all the cards and is manipulating your kindness. If he says ‘jump’ you’ll ask him ‘how high?’. Have some pride in yourself. Give him notice that if he doesn’t change his attitude he’d best find another (expensive) childminder. His choice.

Sorry, but I have to say that your son is acting like a spoilt brat !
You said that it all started after you told him that you wanted a day to spend with your partner rather than be at his beck and call 7 days a week.
There might be a bit of jealousy at play here.
Were you always available before you got together with your partner? If you were, maybe he is having a hard time believing that you do have a life as well as one revolving round him !
Someone that always believes they are right are often very insecure people - if they lose a friend, it's the other person's fault, not their attitude.
Stick to your guns - even if it means that you and your partner leave the house and go out for the day and when he kicks off asking why you weren't home, don't rise to it and just say that you had somewhere to go, Don't even say where. If he threatens to stop the GCs seeing you, don't even reply - he will come off worse if he has to look for an alternative babysitter(that his wife will not want) . If he shouts in front of the children, just speak quietly in return. Hopefully he will eventually realise that trying to bully you won't work

If your son is as nasty to his wife as he is to you, I can't see the marriage lasting very much longer. Your Dil will rely you even more to help with the dgc so you need to stay on good terms with her, whilst standing apart from the DS. He'll soon find himself out on a limb if he doesn't change his ways

I wonder if he’s suffering from stress. I would have minimal contact with him and try to avoid confrontation, you need to be there for dil and children, let him get on with it, if he was rude to me, I would tell him not to, ask if he has a problem, then get away from the situation, you don’t want estrangement like I have.

OP

Your son does seem to have an attitude problem.

In another post, you were overprotective as you did overwrite his feedback and asked your dil who gave you the answer you were looking for

Has it happened more often or is it a one time incident?

You have the right to set up your own boundaries.

Chinesecrested

If your son is as nasty to his wife as he is to you, I can't see the marriage lasting very much longer. Your Dil will rely you even more to help with the dgc so you need to stay on good terms with her, whilst standing apart from the DS. He'll soon find himself out on a limb if he doesn't change his ways

i agree with this, so i don't think you have to worry about having contact with the GC.
your son sounds like an arrogant, self-centred, entitled plank.
doubt his wife will put up with it much longer, nor should she.
just give him a wide berth. don't engage.
and put your partner first. he cares for you and supports you.
your son doesn't care less. he just expects you to be a doormat.

If he can put the phone down on you, then you can do the same to him if he is being aggressive. Just say "I don't deserve to be spoken to in that manner, so I am ending this conversation now" and hand up immediately without giving him time to respond with even more vitriol. If he is rude to your face, then just turn your back on him and walk away - he will probably follow and continue shouting. People like him hate to be ignored. My friend has a son who was just like yours and she used say she would listen to him when he spoke to her with the respect any human deserved, then she would stop what she was doing, put her fingers in her ears and calmly sit down. He would almost explode with frustration, but it eventually did the job and his behaviour improved. He is acting like a 3 year old spoilt brat and needs treating like one.