Help with Saint Bernard's?

so there is this dog, he is only like maybe almost 2 years old, and well something is seriously wrong with him, when he was a little puppy he lost like joint to his legs where he couldnt walk or anything, so my mom and her bf had to carry him so he could go to the restroom or eat, and well its getting more serious,ever since they got this girl dog shes half saint benard though, and ever since shes been around hes more mean, like with people he growls at them and will bite there band of if you come near him, he hates people now, he only likes certain people, when i first met him, he just came right at me attacking me, and then i was around him for awhile, i finally was able to met him and then that night he attacked me again, i may have spooked him cause he was in the dark but when my moms bf came in there he realized what he was doing and stopped at walked away, we have called saint benard rescue and everything, theres no help, but to put him to sleep, everybodys afaid, that he will bite somebody, and i was just wondering if there is anything that can help him, so he wont be like this anymore, we know there has to be something wrong with his brain, but just dont know what it is, please help, thank you.