It seems like a partial retrograde ejaculation
As the mass was near the prostate, removal of the mass would have involved the bladder neck too.Bladder neck is responsible for the semen to flow into the urethra rather than into the bladder.If the bladder neck is compromised during the surgery, there is very good reason to believe that the ejaculation
is going back into the bladder partially which may be responsible for the low quantity of semen.
During prostate surgery the whole bladder neck is removed, and patients usually have nil ejaculation.In this cancer surgery, only the part near the mass would have been removed hence compromising the bladder neck partially explaining the lowered quantity of semen.
Another possibility is scarring of the opening of the prostate and seminal vesicles ducts into the urethra which would restrict the flow of the semen into the urethra.If this is the scenario a simple surgery to open the ducts can done to regain the semen flow.
If it is the bladder neck, there is hardly any thing which can be done as most of the cases it is permanent.There are some drugs used to reverse this condition, but they have serious side effects.
You should discuss these options with operating doctor who will confirm the possibilities.
I hope this answers your question.