I think the easiest word he says is "Hi," and "Skittles" is a little more challenging, so he says "Hi" all the time and saves up his "Skittles"s for when he's really trying hard!
He just chose his first word all on his own, though. We tried to get him to say different things, but he just decided that "Birdie" was his word, and it's still his favorite word today. Sometimes he says it clearly, other times not so much, and he will frequently scream it and it sounds like "URR-EE!!"
He also says several other words, like "Ouch!" and "What?" and "Woof!" and he's recently started saying "Step up" and "Hehehe!"