I think your options are either
- you move into son's room
- he moves into your room
- they get an apartment
Really wouldn't have anyone sleeping in the living room - you, and they will hate it, your house will never feel tidy and they will feel unwanted and in the way. It's too long for a month. Either you give them a room or organise accommodation.
Only you and your OH can weigh up the cost v benefits. Would you be expected to pay for the accommodation if they're coming a long way?
If you have the kind of relationship where you can explain, say to them, he's a terrible sleeper and only sleeps well in his own room. Therefore we've found this lovely b&b or cottage minutes away.
However, if they're flying over, will they hire a car or expect you/DH to run them about? You need to take that into consideration too - if SIL comes too do you have a big enough car to fit 4 adults and a baby seat it, assuming you're going to have to run them about when DH is working.
Even if you love the, dearly, 4 weeks is a looong time, so ensure you've talked through all scenarios with DH and you're both on board with decisions before they arrive.
Eg - who will do cooking, cleaning, buying food? Will they chip in or be expecting you to wait on them hand and foot? Do you all speak the same language? If no, can DH ensure group conversations are always in English so you can join in? Will MIL expect to be kowtowed to as the matriarch of the family, will she attempt to organise your cupboards or pass comment on your slovenly ways?? (All this I've seen threads on recently😂)
Will they expect you as a family to pay for trips out together and meals? Petrol? Obviously there's no tight and wrong here, but you do need to agree and communicate it in advance as misunderstandings over these things have the potential to spoil their time with you.
Good luck, and stock up on gin and chocolate, you'll need it!