Bad tasting post ejaculation fluid

My boyfriend and I have a very active sex life and a few times a week after a normal (and normal tasting) ejaculation I will continue to stimulate him to a further state of arousal.

This results in what he says is an incredible orgasm, however leaves me with a mouthful of bitter, foul tasting fluid. I have never hung around long enough to visualise this, but have shared the taste with him! He agrees its like nothing he's ever produced on his own. I've no idea what this is!?

However,due to the fact this is so arousing for him I would like to keep doing it, but the taste is getting too much now. Is this normal? Are there ways to improve the taste?
All helpful comments would be much appreciated.