OK...Don't scare the kid, he wants to do them, so he's gonna do them, you're making it seem like they're gonna kill him. I did shrooms when I was 13, granted, I was 5'7 and already had hair on my chest (fun to trip out on, hahaha). Mushrooms are fun. Mushrooms are fucking fantastic. They are my favourite drug ever.

For your first time (and your size), do 1.5 grams, of dry cubensies (blue cubensies, mexican cubes, blue mould, whatever your dealer is calling em... they are kinda whitey-greyey-blue and smell like pooey death) and if you dont feel anything, wait a week or so, then try 2.5-3 grams... MAke sure you have a house (with no parents/nosey neighbors, etc. for the entire day) to chill at for your first time, preferably with a backyard that you can go "explore", because nature is your best friend on mush. It is impossible to OD on mushrooms I do believe, you can't fit enough in your stomach over the period of a trip to OD. I've done a half ounce dry, and I've seen people do a full 30 grams of medium quality mush. Make sure you have lots of weed to smoke! especially for the comedown, otherwise you will feel kind of sick. Personally, I don't get a hangover at all, and none of my friends do, your mind just feels a little overwhelmed the next day, but its nothing unmanagable. It's good that you want to do it, keep that attitude and nothing will go wrong. Most bad trips ive seen are because of the governments misrepresentation of drugs... e.g. "am I going to od? Will I never be the same again?" don't worry about ANYTHING like that. remember, tons of people do shrooms, and 100% of the time, its works out fine in the end.
That said, read up on www.erowid.com about psilocybin mushrooms, and lears as much as you can, because you should never take one persons advise without other sources, although my information is accurate...