Good intention, but bad idea. Instead, keep a scrapbook for him. To include text, write it in Emacs then print it out and paste in the paper. Put pictures in the scrapbook too. Involve your wife and decorate the pages too, if you like.
Forget about using gmail for this. Keep his childhood private -- he'll thank you for it. The scrapbook is something you can look at together on the couch as a family.
I don't know about using a scrapbook... part of the point is to make it personal and frequent. I know that my wife has tried to keep up with a baby book, and it's hard to know where to draw the line between 'scrapbook-able' events and just plain events. The line for email is so much lower that you're going to capture more.
Then instead of sending an email, just append a bit of text in a text file. Separate those bits with a few blank lines and a "----" or something.
Keep a text file for each month, or week. Keep a directory for each year.
If your wife has something to add, have her either add to the text file herself, or else have her just type it up in gedit and you'll take care of moving the text.
Putting every little thing about your kid's life into emails is a recipe for disaster. Google will know more about him than anyone should know about anyone. That's obscene. Really. If you don't believe me, email Bruce Schneier or somebody and ask what they think.
And, besides, if you did email every single little thing to him, could you imagine the headache of looking through all those emails? I know from experience how difficult it is to clean out my inbox. Could you imagine a whole childhood of them?
Forget about using gmail for this. Keep his childhood private -- he'll thank you for it. The scrapbook is something you can look at together on the couch as a family.