Did the same thing with my boy last year. This is a video of him a month before his 4th birthday. Love these little bikes. I bought him a new crf50 with training wheels but it's a little big for him to learn properly on at that age for sure. I hear ya about the proud parent thing, give the little guy a pat on the back from me, looks like he's doing great!
http://youtu.be/3lzznf6K37M