I guess it's best to start with a small mushroom to go "into it", once the effects start (30 minutes) he'll feel a little uncomfortable, but then it's time to take more, after a good hour he'll be tripping, and then it's just up to him how far he wants to go, but if you go slow, and slowly build up to a desired state not much can go wrong.
Actually, the most uncomfortable is the beginning, i have noticed that when you take too little psilocybin it will keep you pretty disconnected for a good wile, and when you take a stronger dose it will take you to the next level much faster.
Building up slow is a good idea imo, after that i can't say, it depends on his sensitivity to it, and how much he ate etc.
If he's experienced with other psychedelics he can handle a good dose, if he's not he better take it easy, you don't wanna spook him ofcourse.
I guess a gram of dried strong mushrooms is more then enough for his first experience, if he likes it, he can dive a little deeper.