Your husband isn't good at planning holidays. If he were, you would all be enjoying yourselves.
What he has done is plan a holiday for his parents, because he loves them and they're old and he's worried he doesn't have much time left with them.
But you and your DS are bored to tears. You're in a boring village with nothing to do and the only things on the agenda are going for walks (with shitty sandwiches, not even a pub lunch) and watching the soaps in the evening.
You need to be brutally honest with your DH and tell him you can't bear another holiday like this.
Actually, I would tell him now, in the hope that you can salvage something from this holiday. Tell him it's time you made a plan to drive somewhere there is actually something to fucking do, and have a nice lunch in a pub or restaurant. If your in laws want to come, they are welcome. If they don't, they can go on another walk with their soggy ham sandwiches and you'll see them tonight after Corrie. Because yes, it's nice for them to be included and spend time with their son and grandson. But it's not nice for them to basically force you to dance to their incredibly boring tune every day.
Maybe they don't realise you're so bored and think you're having as lovely a time as they are. But your husband must realise. And you and your son deserve a nice holiday as much as they do.
Perhaps in future you could do the odd weekend away with them and have separate family holidays that you actually want to go on. We're going away for two weeks and my parents are joining us for a long weekend in the middle. Maybe that could also be an acceptable compromise for the future, if your PIL are capable of travelling on their own. If not, stick to weekends.
The problem is not family holidays. The problem is YOUR family holidays.