my sisters 5 year old jack russel has started biting people

by kara
(hertfordshire)

my sister 5 year old jack russel has started biting people, she has got 3 kids and have grown up with him and he has never bit them altho he did growl at her olderst son the other day whom is 10. he bit my grandad when he went to stroke him and drew blood and had to go to the hospital and then this morning the milk men put his had in the letter box to post something and he bit him (ok well i dont think he should of put his hand thro the letter box)my sister phoned the vets annonomously ad they saidhe would have to be put down. this has really got her worried as he has never neen the boss my sister has and he will do anything to her command.
any suggestions please