When a man is ready to ejaculate his body will undergo a series of events.
He will see an increase in the size of the head of the penis and the head may also change to a purplish color. His Cowper's gland will secret 'pre-cum' fluid which dribbles out of his urethra. His testes will move in the direction of his body, and increase in size. As well, he may experience a body flush, muscle tension, increase in heart rate and rising blood pressure.
Just before ejaculation he will feel contractions in his vas deferens, seminal vesicles, and the prostate, causing seminal fluid (ejaculate) to collect in a pool at the base of his penis. He will feel a 'tickling' type sensation. When ready to ejaculate, he will feel a "throbbing" around his urethra.
Ejaculate will leave his penis at roughly the same rate of travel as a city bus, about 28 miles per hour. But can reach speeds of 43 miles per hour depending how long since the last time he came. (It's all fun and games until somebody loses an eye !!)
Once he has ejaculated, his scrotum and testes will return to normal their size. He will have a general feeling of relaxation and experience a refractory period (where a he is physically incapability of getting anotherection). This period may be from a few minutes to much longer depending on his age.
Sex Tip: Without he is at the point of no return, you can stop him from ejaculating by firmly (yet very gently) squeezing on the tip of the penis.