Can you set up playdates? I didn't have any kids in my neighborhood to play with my 4 yr. old and she would get so bored, there's only so many activities I could do with her. I finally got in touch with some preschool classmates and started setting up playdates.
I know that may be a lot for your MIL BUT 5 yr. olds can usually play together with out much assistance. You could alternate homes. My 5 year old LOVES it, they just find things to do together including playing in the backyard.
Also, go to a teacher supply store or look around target, wal mart for teaching books for pre-k, kindergarten etc. There are fun learning activities in there. You could have him do so many pages a day when you are gone.
Buy age appropriate crafts for him from Michaels. A big sketch pad, washable paints, play doh. I am sure you have a lot of toys but unless you pull them out and put them on the kitchen table, sometimes they sit in drawers and collect dust. Maybe alternate different crafts every day.
Tell MIL that the TV is limited every day to a specific time. OR have your child pick 2 or 3 shows that he can watch and that's it.
Ask your MIL if she can go for a walk every day, maybe to a near by park OR she can drive the kids to a park.
Vacation bible schools are offered at most churches and usually are super CHEAP. Sign him up....even if it's not your church, sign up for several. If you can afford camp, do some camps.