I agree that there's nothing magic about the first birthday, it's just a conservative guideline for when almost all babies are ready, when really some are ready earlier. We started earlier with my DD. Like the earlier poster recommended, we started with a lightweight knit blanket that my aunt made for us--it had enough holes in it that, if it did end up over DD's face, she would still be able to breathe. We learned with that that if the blanket DID end up over her face, or tangled around her in an uncomfortable way, she would DEFINITELY wake up and let us know about it! So we actually went back to waiting a bit longer, not because it was unsafe for her, but because WE didn't want to get woken up for blanket re-adjustment 3 times a night!
Like the earlier poster said, if your son can move himself around and pull a blanket off of his head (try this for peek-a-boo when he's awake), then he's fine. Also, if, like my DD, he'll holler whenever he's uncomfortable, you also will know that he's safe. If, like my DD, he tends to get tangled and holler, I still recommend the "sleep sacks." At 21 mos my DD still wears an XL one, and LOVES it. Much to my surprise, she can walk around the house with her feet in the sack--she just loves how warm and cozy it makes her. GL!