He is either very unique, or there is something wrong.
The male multiple orgasm is normally no ejaculatory. Once he ejaculates it is over. That is what tantric sex is all about, getting him super aroused where he will continuously orgasm, without ejaculating. Normally there is a refractory period after an ejaculation which gives the blood vessels in his penis time to recover from the stress involved in erection and ejaculation. He gets super sensitive to stimulation during the refractory period. He may experience touch as painful or ticklish during this time, especially right after ejaculation.
Is he using an ED drug (Viagra, Ciallis or Levitra)? It is possible to have multiple orgasms in short succession using these drugs. They shorten the refractory period to the extent where it may last only a few seconds (like the female refractory time).
The fluids are stored in the prostate and seminal vessels. He does not have an unlimited supply of sperm. So every time he ejaculate, there are less sperm available. So it will get thinner and less, to the point where he may dry up almost altogether after a while. He will however regenerate millions and millions of new sperm soon after. A man is not like a won am where we are born with all the eggs we will ever have already in our ovaries. Men's bodies make sperm continuously. Older sperm dies and go bad, so he needs to replenish constantly.
Take care!