How to stop barking when alone

Hi, I adopted a dog abouth 3 weeks ago and have never heard a peep out of him. Never heard a bark, growl, anything and it just came to my attention from a note from the neighbors that he barks when he is alone the entire time. Now he is only alone for one or two hours at a time during the day, but of course there are the occasional longer periods. I can't have a dog that barks all the time because I live in apartments. I know they can take time to have effect but I bought one of those comfort zone plug ins to see if that can help. Other things I have tried is giving him a kong that has been frozen with wet dog food inside. When he is alone he ignores it, I come home and its completely full but then he starts to chew on it and it will keep him busy for hours, just not when he is alone. We block him off in the living room when we are gone. Crating does not work, he tries to escape to the point where he will hurt himself, broke a few crates. I have also tried putting a blanket of mine in the living room for him to lay on and then giving him a tasty natural bone, the one with marrow and stuff inside to chew on, he goes crazy for them, but again I don't think he touches it while we are gone. He is a really low energy dog, part bulldog and sleeps all day normally. He also gets let out plenty. We go to the dog park 3 sometimes 4 times a day. Any help would be sooo much appreciated. If things don't improve in the next month I may tried one of the citronella bark collars.

Also wondering about the citronella or collars that make a sound when barking. Do you need to be around to train them with it? Like I said I have never heard my dog bark before. Never ever. Its so weird. So I don't know how to train him or tackle that problem. I don't even know how to get him to bark. I also never catch him barking because he must be able to hear really well because he is quite as I am walking up to the apartment. But I talked to the next door neighbors and upstairs neighbors and they say he does bark the entire time he is alone. Even if its just a short period of time.

I am having a similar issue with one of my dogs... however... I've had him 4 years now, and he's just decided he likes the sound of his own voice....

I am looking in to getting a no-bark collar for him. It's not my first choice, but I am out of options for him, and the neighbors are tired of it (understandably!)

After doing my research, I've decided AGAINST citronella and sound collars. Dogs have VERY sensitive sniffers and ears, and a foul smelling spray and high pitched noise may seem like a more 'humane' method - but I do not agree. I'm going to go the traditional route with him. A short buzz from the collar seems like a better option than a lingering smell or sound to me.

 My only problem is he does all the barking when he is alone, and I don't think I would trust a shock collar on him unsupervised. I do believe they are very effective when necessary but I would feel horrible if something went wrong while I was not home. It seems like there would be less chance of physical harm from a spray or noise

If he's barking consistently while he's alone - that would appear to be more separation anxiety based to me. There's a REALLY good book out there by Patricia McConnell called I'll Be Home Soon - it's not expensive at all - and REALLY REALLY GOOD! I would try to desensitize him to you leaving/coming home, and work on making his time spent at home alone more enjoyable for him.