Not knowing details...and just guessing....a question might be...how long does it take him to ejaculate when or if he spank the monkeys?
If he is not very active, it is possible he needs more stimulation (like was said).
He may also be pre-occupied with other problems.
Another possibility....he may be SO active that there isn't enough of a supply.
A remote possibility...his prostate could contain a very thick semen, for one reason or another, that is just too thick to ejaculate....
Above all....talking to him is the biggest help. It is a problem with him and not you.
He, ultimately, may have to see a doctor/ urologist for a proper diagnosis if this continues to be a problem.