How large is your grandson? Can you alter this pattern to fit him?

Or you can try altering a sleeper pattern or sewing together a shirt and pants.

I found this sketch on a site discussing costumes--it may help a little, too.
http://www.swtroopers.com/coveralls/coveralls01.jpg

Here is a link that might be useful: Coverall pattern

Thanks Dian for your suggestions. I love Quik Sew patterns and have used them often, but I think that particular one would be too big, although it's exactly what I want. I think the idea of combining a shirt and pants is a good one. My DGS is coming to spend the weekend, so I'll get some measurements and go from there. Thanks again.