Hi, my previous dog started to suddenly do that and I understand how distressing it is. First of all, try to shake off the feeling of having failed him, there are lots of reasons why he might be behaving like this, plenty of dogs have to be left for a few hours every day and cope quite happily, so its doubtful that you're to blame, the main thing is that you're seeking a cause and a solution. Its hard to pinpoint a reason from a distance but the behaviourist will be able to take in the whole picture and hopefully work with you on finding a solution. I would advise you to try and stay as calm as you can when you leave him, its understandable that you're now anxious but Oscar will be picking that up and the more fuss you make, the more stressed he'll become. I don't kniw how your Dad reacts when he goes to pick him up when he's barking but a calm approach is useful there too, i.e. not too much "sympathy" and OTT greeting but try and keep both departure and arrival as low key and matter of fact as possible. I hope the behaviourisf has some advice for you, do keep us posted. Best of luck!