You say "His DP’s parents - they have taken over quite a bit, insisting on doing things that need doing in his house. Granted, his DP asked them to. When they said they would do it, my son said that ...No, he would do it when he felt ready and had the time. They then insisted and basically took over. My son seems to be letting them walk all over him and I have said nothing because I feel as an adult, he needs to sort it out himself. It’s his business and not mine.
However, they have suggested that I could help by doing what his DP suggested. I already offered help to my son ages ago concerning the same thing but he said no he would do it.
I feel a bit insulted and feel they are taking over how my son should run his life. AIBU or are they?
There seem to be two camps here - you and your son versus his partner and their parents, two completely different power hierarchies - and believe me, there is a power struggle going on here.
I suspect that your son's partner would like to have things done around the house that your son for some reason doesn't want to do. Her parents are aware of this and ready and willing to do them, so she has either called on their help as more powerful people than her, or they have stepped in themselves, again feeling themselves more powerful than this young whippersnapper who isn't providing a home fit for their daughter.
In his own house, power should be shared equably between your son and his partner, not reinforced on one side by intervention by outsiders. You know that and have been practising it, but her parents don't, and are still treating her as their helpless little lamb.
At some point your son is going to have to rebel, and tell his "inlaws" a few home truths. That may cause a rupture with them, and potentially the end of his relationship with their daughter. I reckon he needs to do it soon, and tactfully, before he disappears completely into their shadow.
He also needs to do some of the things that he has been putting off, if they are important to his partner. Even taking on one thing would be showing willing. Perhaps you could mention to him (and to the inlaws if the subject comes up again) that you would be happy to help him if he wanted you to and specifically asked you to but it is his home and you would never interfere with his and his partner's plans.