There's a free M4 morph for a teenage boy at Poser Addicts..I just got it, looks pretty cool. I'm not really familiar with their products, but I've liked what I've seen so far. There are also a number of very young morphs in the banner at the top, so maybe that will give you an idea. My thinking about using a male character other than M4 is that M4 lovers complain that he's not well enough supported in the marketplace, and there's by far more for m4 than other male characters from what i've seen. So I'd say, depending upon the content you need in terms of clothing and so forth, check out whats available for the character you choose before you begin.

http://shop.poseraddicts.com/index.php?act=viewProd&productId=3625