I can play guitar & my brother wants to learn; I've drawn out a few chord charts for him, & what I think would be good now would be for him to play along with a few songs. I was thinking 'Mr. Postman' by the Marvelettes as it has like 4 chords in total.

Could anyone recommend any other songs my brother could learn?

Check out justinguitar.com. In addition to a well-thought-out series of lessons, he has a selection of songs (along with tutorial videos for each) that are sync'ed with the chords and lessons covered at each level. He has a pretty diverse range of styles, so there's usually at least one you'll like and / or know out of each group.
Outside by Staind
Hollow by Godsmack
Roses are red
Voilets are blue
The only bulge in my pocket is my wallet
No i'm not happy to see you
Quote by StupidGenius11

Go on, my son.

Why, that's not even a good I - IV - V progression. And it only sounds good if you play it as a I - II - VI. If you want to learn three chords and form a band, you at least have to learn G, C and D.

How about Van Morrison's Brown Eyed Girl.

These songs use only one chord:

Aretha Franklin - Chain of Fools
Harry Nilsson - Coconut
The Beatles - Tomorrow Never Knows
When I started playing guitar, someone suggested Metallica's "Enter Sandman". I don't quite remember what their reason was for that, but I do think the mix of picking and power chords makes for a useful exercise -- if that is a direction your brother is interested in going in, of course.