[mom2M&A]

First of all I'm sure everything will go fine. Your MIL will probably try to be pretty careful about routine so she gets another chance sooner rather than later.
But...my experience is that bedtime routine will change a little (exp. we got to watch tv and eat popcorn AFTER bath w/grandma!), so just write down the usual routine and she can make sure everything gets done.
Times for lunch and dinner are important and any snack times, but just suggest menus. I usually couch it as we eat at x and x so if you start to see whining he might be hungry...
Your son will be worried about when you will get back, so you should have a calendar or explainations of when mom and dad will be back. Also set up some time for phone calls.
also if it's like our house where turning on the tv/vcr is rocket science leave directions for that stuff and where favorite videos are. my mom couldn't work my oven and I got a call about it. Try to do that over the phone!