Mary, I have to go alone here. I would pick #2. First I love his face here, I think a formal portrait is something the child will want to hang as an adult - since I married "Dumbo" as a kindergartner (his ears have grown to his adult head size :)). I think the first one with all its personality will not be a favorite to him. I think if you do a three quarter of #2 and do a blended background, meaning lose the detail, keep some depth and light, it could frame him nicely. I think his expression is sweet for a piece of art instead of a photo to send to grandma. Can you blow these up to see shadow detail on the faces?

Well, the mother has seen these, and-
 

Her very favorite, is #2 the formal one in front of the pampas grass, she says she likes his expression the most in that one. Her second choice is I believe #4 (the second one in the tree facing right)

I asked about retaking the pictures and she really moaned and groaned about it. We had a TIME getting him to behave for these.

She doesn't care what the background looks like so I can certainly manipulate the grass or the tree to some degree. I know some of you don't care for number 2, but for those that do, do you think it will would ok size wise on a 24X36..3/4?

Mary, I think you could do it either way... but personally I would blur the background, like you knocked it out with your depth of field while shooting. Interesting the mother wanted formal and dressed him in overalls. ;)