Is this happening just only in the recent days or it has been always like this? You can train your dog to be quiet if he is having barking behaviour all the time but it maybe very harder for 5 year old dog.If it got this issue recently then there might be any medical issue in its body so in that case you need to visit a vet as soon as possible.
Re: training your dog to be quiet, I agree. But if you don't know how to train your dog, then hire someone else (or a professional) who knows how to do it instead.
Re: possible medical issues, it's totally possible and this should be the most realistic thing to do, before even considering "putting your dog down."
