Hello.
Puberty occurs at different ages in different people, and age 11 is certainly not early. Your son has just had his first ejaculation. Hurray! Celebrate with him that he's now an adult, and also talk with him about the pleasures of sexuality as well as the responsibilities.
If you haven't already done so, now is a good time to talk casually and nonjudgmentally about sex. He'll need reassurance that self-pleasuring (masturbation) is part of life and not harmful, and he'll also need to know that, just like driving a car, he's still too young to take responsibility for being sexual with someone else, but when he IS old enough, he needs to be knowledgable about risk reduction, etc.
There are some excellent books for his age group. I recommend "Ask Me Anything" by Marty Klein and "The Family Book about Sex," by Dr. Mary Calderone. Dr. J