You stand correct that a male can achieve an orgasm without
ejaculation.  Male "orgasm" technically refers to the rhythmic
contraction of the prostate gland, with or without a concommitant
ejacaulation of semen---although the two usually occur together.  
Direct
stimulation of the penis (by masturbation or intercourse) triggers a
chain of responses in the male, including emission of sperm from the
testes, up towards the seminal vesicles which add fluid to the sperm,
and then towards the prostate, which adds more fluid (providing the
sperm some nutrients and ensuring the vagina will not be too acidic to
kill them).  Ultimately, at the point of 'climax,' the prostate gland,
as you have astutely observed, contracts forcefully---and
pleasurably---, pushing semen up the urethra and out the penis.  It is
this contraction itself, and not the actual emission of semen, that is
pleasuable "orgasms."  You are also correct in that direct stimulation
of the prostate gland, either internally or externally, can cause the
prostate to contract---once again, pleasurably---, causing orgasm.  
This
orgasm may or may not be accompanied by a simultaneous ejaculation. 
There are probably two reasons why ejaculation might NOT occur during
such stimulation (although here I am not 100% clear)---1. direct
stimulation of the prostate may not be the same as the stimulation is
receives during a usual sexual cycle, in which stimulation starts with
the penis (i.e. may not be as effective), and 2. there may simply not be enough sperm and/or prostatic fluid to create semen to be ejaculated.


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