i think this is no problem.
I don't know how much it variues from man to man, but as a man, I can tell you the volume of the semen that I ejaculate varies from orgasm to orgasm, even if it's been a few days since I've come last. Sometimes getting distracted during orgasm causes the ejaculation to shut down, so there's less. Did he pull his penis out of you before he came? If so, there could have been enough of a distraction to make him shoot less.
There is no right amount. It varies from man to man.