My guess is that it either caused severe pain once and now he hates it or too much of an ordeal is happening and he has learned to get worked up..
How long does he have to be on these?
I feel so bad for him (and you)

. The most restraint I have to use when giving fluids to Ellie is to hold her by the scruff area so she can't walk away. Oh how often do they have to be given?