You might try hex paper (hexagonal graph paper), you can download & print it for free here: http://incompetech.com/graphpaper/hexagonal/
Try a scale of each hex is 10ft, or 5ft.
If your grandson doesn't already know how to interpret map images, you'll have to explain them, but you can make it easy by drawing in things like bushes, trees, driveways, sidewalks, etc...
On the other hand, part of the fun/challenge is to figure it out with all the ambiguities that a pirate treasure map comes with. 10 paces (kid's paces) from the bushes by the mailbox due east (great time to teach simple compass use), then 20 paces north for the next clue. Maybe a simple riddle or 2 for a clue: "When the sun shines on the red flowers, the next clue is at your back." And the clue is in the shed behind him when he stands in front of the red tulips... You get the idea. Ever watch the TV show Blues Clues
? or Dora the Explorer
? They use simple maps & clues in every show, might be a couple of good ideas there.
You can place the clue under a rock, or taped to the branch of a tree; kids love that sort of thing, each clue helps to build excitment, especially if there is some small reward (like a chocolate coin, plastic pirate sword, etc...) for finding major/tough to find clues.
Geocaching is a fun thing to do, much like a treasure hunt. When he's older, maybe a "fox hunt" (Xmitter hunt) with a ham radio club would be a fun thing for you both to do. Hope you both have fun.