If he started being this way about a year ago then something must have triggered it. Can you remember to around a year ago or even farther back to weird behaviours he might have had but was never noticed or any situations. your dog is 3 and this started a year ago so for 2 years he was fine. dogs don't just change their behaviour for no reason, something triggers it such as a bad experience. Also, is there something that triggers his bad behaviour or sets him off? I know this might be difficult to pinpoint but there is usually something which could be anything from a certain situation, sounds, smells, time of day, certain person. Really look hard at his behaviour and how he acts and even try to document his mood changes and what was going on when this happened in a journal. This could give you good insight and maybe show some kind of pattern. also, is he aggressive towards you too or other people?
It is always a very sad situation when an owner has to consider putting their dog down but I think that you should try everything possible untill you can't try anymore because you obviously love your dog. Call every trainer and specialist you can, look up trainers and different resources on the internet, ect. Have you had him checked out by a vet to rule out medical reasons? Sometimes behaviour changes are caused by medical problems. Does he get a lot of exercies? Dogs that are aggressive need a lot of exercise to release extra energy and tention. Also, how much time do you spend with him? If you don't spend a lot of time with him he could be acting out and getting rewarded for his bad behaviour; he may have learned that bad behaviour is working to get what he wants which has now gotten out of hand. Try re-enforcing possitive good behaviour. When he acts good and is calm and submissive give lots of prasie, treats and attention and when bad give as little attention as possible but correct it right away by a time out in a 'naughty place' or crate. Also, how are you with leadership? Are you the "pack leader" with your dog or does your dog control you? This also causes bad behaviour, aggression, and acting out. A dog will take any attention they can get whether good or bad. Try establishing a more dominant role with your dog. You can do this by practicing obediance at your house with the basic commands such as sit and stay is a really effective one. Do not let your dog get away with unacceptable behaviour because if you do then they learn they can and they don't have to listen to you; this teaches them that you are the leader and obedience.
Most behaviour is changable but does not happen over night. It is especially difficult with large aggressive dogs such as yours as someone can get hurt so Remember to take the precaustions to ensure everyones safety. Consistancy and possitive re-enforcement is an effective way to go. All you can do is try and trying is better than doing nothing or giving up
