Just tell him animals can be pets and still productive! After all, you have no problem taking away all their nice little eggs, which are potential babies, the loss of which could cause them serious stress....
Most farmers I know love their animals and hate to see them suffer. Although they easily take them to the abbattoir at the end of their allocated livespan, they'd still hate to loose them, say to foot and mouth, or a flood or fire. You do have to be careful though if you befriend some larger animal in it's youth - say an orphan calf, that grows up into a massive big cow or bull and then still wants to play with you as it did as a sweet little calf! They don't understand that your strength hasn't grown the same as theirs... That can get dangerous.
Well, not much danger of your hens running you over, though.
Ina