As ChunkyPickle says, set the tone from the beginning. We live in Oz and have had a number of long term visitors from the UK, including the ILs. (Last year 3 months and this year 8 weeks - far to long if you ask me.)
Don't go out of your way to plan day trips for them. Your number one priority is your DS and if they had really wanted to see the country, they would have made other plans for themselves. My ILs are of the type that will not go off on their own to explore and so we are with them 24/7 from the day they arrive to the day they leave! Their main reason for coming all this distance is to spend time with their grandson and they are very happy doing this.
Once they arrive make sure that they know where things are to make a cup of tea or a snack (toast and jam is good) and explain what you want them to do with the cups and plates when they have finished. I tend to make it clear that lunch is a 'do it yourself' affair as I am often too busy with DS to deal with guests until it is too late! Don't be afraid to ask for help at any time that you are feeling overwhelmed; if they are normal people they will want to help.
An expensive mistake that I made in the early days was cooking to impress; preparing nice dinners just because we had guests. Just cook what you would normally do, even if it's baked potatoes and baked beans! Maybe your DH could explain to them that money is tight and ask if they would mind contributing an amount to the grocery bill. We normally go halves minus nappies and foods specifically for DS.
If there are certain things that you do to save money, eg waiting for a full load to do washing on economy 7, short showers etc, explain this early on. It can save a lot of resentment building up.
If there is a certain time of the day that you find stressful, try to use the IL's to your advantage. For example if the hour before dinner is particularly difficult, make it a daily granny and grandpa playtime with DS where they can take him into the garden or read him stories.
Finally, keep the camera in sight and make sure that you get lots of photos together. We never remember this and often end up doing the 'family shot' just before we leave for the airport.
Approach it with positive thoughts, you may really enjoy it! Good luck.