AIBU or are they?

What exactly have the parents been doing and what exactly are they saying you should be doing? Some examples would be helpful to understand things better though they do seem very domineering and I wouldn’t want them as my in-laws. Also you say they are a different generation, are you a lot older than them? I assume you’re a lady or if they expect you to do work are you a gentleman? Sorry just trying to get a feel for things not prying or being rude.

I had a very dear friend, sadly now dead, who acted in exactly the way the girlfriend' parents do, for what she saw as the best possible reasons; she and her husband rushed to do far too much for their adult children in terms of both practical help (decorating, gardening etc) and financial assistance. Their adult children were far too dependent on them, and it did cause some friction with the parents of their partners, who felt as you, (and I) do, that adult children should be more responsible for themselves.

(posted too soon.)They expected the co 'in-laws' to match their support, which created some tension.
Stand firm if they raise the matter with you, and tell them why directly.
Equally, I sympathise: one of my children is living very happily with a partner who for various reasons won't commit to marriage; my worry is that there is no security in terms of the home, which belongs to the partner. I say nothing.

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.

This post caused me some confusion, especially working out what DP meant? (I am guessing it means ^Darling Partner^)

Thanks all.

They seem to be very over protective but probably mean well. It is probably not good for your son that they are doing all this, not good for his self esteem or motivation,either. I would keep right out of it because sooner or later there will probably some resentment coming in .

Bashful
You are not the unreasonable one, they are!!
They need to know that your son and his DP can do things their own way, when they're ready to. But I get the feeling they haven't let go of their 'child' and the child is still hanging onto the apron strings, in which case your son may be allowing them to do things 'their way' in order to keep the peace!
Also, it may be their way of trying to help out knowing their child doesn't pay anything (which DP really should, as it is a partnership).
I think you should make time with your son to discuss what's happening, and this is not interfering. Tell him you're worried he's being taken advantage of and you'd like his views on the situation. He may want some help but not know who to ask, or how!
He may welcome your views, but he could also just tell you you're overreacting, but you definitely need to have a personal chat with him!!