I'm 'traditional authority' adverse. Where decision making is placed due to reasons not relevant to skill and training. If your dad is like that then I'd say that 'we are doing it this way based on what the vet recommended.'
But if your dad just doesn't want to do anything he's asked to do then I think the only sane thing to do is not to ask him to do anything. Better to admit to yourself that he is, in the literal sense, unreliable so you have hopes of getting the tasks done among yourselves.
That's no excuse to let him be a mooch, either. If he's able but unwilling to help then he shouldn't receive the benefit of your work either.
But if your dad just doesn't want to do anything he's asked to do then I think the only sane thing to do is not to ask him to do anything. Better to admit to yourself that he is, in the literal sense, unreliable so you have hopes of getting the tasks done among yourselves.
That's no excuse to let him be a mooch, either. If he's able but unwilling to help then he shouldn't receive the benefit of your work either.