MIL has forbidden DS to call for his friend when we visit

No they are not, just when DS is at GPs. They go to school together and live in walking distance, they can see each other plenty apart from the 2 days a month DS visits GPs

Maybe suggest he must spend an hour with grandparents and meal times with them but then allowed to play with friend x

When my son was about 6, he became very close friends with a boy round the corner, and they were always at each other's houses for sleepovers. I'm not sure what the attraction was - except he told me their house was much tidier than ours! - but they got on really well and had great fun together. I think friendships at this age can be very important, and I wouldn't give in to the in-laws when, in practice, they're asking you to stop him seeing his friend.

This indicates that you could take some things with you to help your DS interact with GP and small cousins.
Specific games, puzzles or activities could assist them to engage happily for an hour or two.
Could you instal a swing, sandpit etc.
It is rude for your DS to go next door when his GP and cousins would like him to be with family.

I agree with MIL.

This, im really surprised by the initial responses from people saying this is ok, its really rude if you are there to see grandparents if he just disappears off?! Its ok for kids to NOT be optimally entertained occasionally. There are other kids at his grandparents he can play with, its not as tho hes forced to sit there with literally nothing to do.

Id just reduce the visit down to a couple of hours. It's the easiest solution.
Why are you spending all day every other Sunday with two people who sound a bit miserable anyway.
You could be doing something better with your time.

This! They sound like right miseries. I'd say that doesn't work for us, we'll give it a miss then.

Net mummers? 😳🤣

We have long visits because they stop over due to distance - it drags at their house or ours despite everyones best efforts. It's a long period of time for such young kids or even near adults we have now- you may just have to get better at leaving.

I think playing it by ear at length in future and having more weekend with sports will all help.

But yes a few hours playing with neigbour in such a long visist seems fine but clearly isn't want they want. We tend to do walks out to break things up.

After reading your updates, OP, I think the visits are simply too long, particularly with a grandparent so inflexible that they take umbrage at a six-year-old going to play for a couple of hours during a seven-hour visit.

I'd tell them in advance what time you'll be leaving and stick to it. You can make plans for afterwards if you need an excuse, but I wouldn't even bother. I'd simply say, "It's a very long day for the kids, so we'll be heading off at X."

'"it won't kill kids to be bored while visiting the grandparents" are the lazy type who cause the boredom in the first place. I've never heard it from anyone I consider interesting and engaging!'

The fact you’re able to generalise about these people so confidently suggests this is a topic of conversation you have with unusual frequency. I can’t say it comes up often among the people I know, and certainly not among anyone I’d describe as particularly interesting or engaging.

Why would it be awkward to not be allowed in MIL’s garden?

Yes, indeed they are. I read the OP in full. But thank you for checking.

For context we are there one day every other weekend and the visits are the whole day - about 10.30-5pm or later.

This sounds absolutely hellish!

Who has time to spend an entire day in someone else’s house, every other Sunday?Surely you have other things to be doing OP?

I’d be offering them 10am to 12pm or 12pm - 2pm, maximum. If they want to spend any more time than that with their grandson, they can have him on their own until 5pm. The entitlement to your time is unbelievable!

As a dc my dgm's ndn had a dgc called Margaret .
We were great friends.
As an adult with my own dc we walked into a cafe in our home town and there was a grown up Margaret!
Recognised her immediately and hadn't seen jer for at least 20 years!
Your ils sounds really bloody mean..
They reminded me of mine. Mil regularly took toys off baby ds and declared he should be happy just sitting on her knee.
.

'If necessary perhaps we just won't stay as long, or let my son go over just to the neighbours garden at the very end, again specifying that they aren't allowed in the grandparents garden, but they can come to our house to play at a time when we arrange.'

That sounds like a really good compromise, a whole day sounds a bit too much for him.

I think this is very sensible OP.

I think people are giving you a hard time about entertaining your DS, but I know I find it much harder to entertain my DS at my PILs than at home or at my parents.

My parents are in their 70s try and make an effort and have a few toys around etc. and are happy for DS to go off and play. They don't play with him all day long and I wouldn't expect them to, but they will listen whilst he talks about Minecraft for a bit.

Whereas my PILs are in their 80s and want DS to sit at the table with them whilst they talk at length about acquaintances/distant family members who me and DS have never met, and DH might know a little. In particular, it's often focussed on which elderly acquaintances have passed away and what health conditions they had. They are lovely people but honestly I struggle with the visits and I can tell DS struggles too.

I can see why it is especially tricky if you have a 1-year-old too and the visits are so long and frequent.

They are friends at school, they can be see each other in OP's own time. It's not like they live miles away and only see each other a couple of times a year, they go to school together

My own family would be an open conversation, this is boring let's get a game out.

DH family, would be a different kettle of fish. Would be best dealt with by reducing visits.

I think children should be encouraged to make and maintain friendships and therefore I’m afraid I think your IL’s are being incredibly selfish here.

My children, their spouses and grandchildren would have no problem in telling me that they are bored. It would be a statement not an argument and we would all need to do something about it. It is just who we are. We say what we think and I prefer it that way. I want my grandchildren to feel relaxed and happy when they are here, not to have to worry about being on their best behaviour. I certainly wouldn’t mind if they invited friends over.

I don't see the point of kids being bored at Grandparents.
We have one set that fully engage with the kids the others are almost kids should be seen and not heard, let's sit in the room with 4 seats.
So then they wonder why the teens sit on their phones. The day is coming when thsy decide to stay home.