
It is possible to work on SA while you have to leave the dogs, it just makes it a bit harder, that's all.
PET remedy diffusers are supposed to be very good (significantly better than DAP diffusers, if all the reviews I've read and hear are anything to go by although the DAP collars still seem to be good), and I've spoken to you about melatonin already :-)
The basic method for SA is to start by leaving the dog for an amount of time he's happy with - even if it literally a split second with you stepping out of sight then straight back in. Calm praise and repeate, or perhaps repeat later on and just do tiny bits of training over the day. The time then gets built up slowly.
Another method is to have a unique object that the dog never sees, and in staged sessions, put it out in clear sight then ignore the dog for a little bit: the idea being to teach the dog that this object is a cue that he is going to be left to his own devices for a little while. Started with you present, it can help teach some dogs that it's a time for them just to chill, not to panic. As with the first method, you would build up the time that the object is out and he is ignored, but starting initially only with you at home, then progressing to tiny amounts of you stepping out then building up the time again (you would need to drop the time to begin this stage - so if you had built up to 30 minutes say, with you present, you'd go back to one minute of object in sight and you disappearing - as you raise one criteria (you leaving), you drop another (duration)).
A third method - my personal favourite - is to begin giving him special time with something to do, such as a filled kong, on a special spot (new dog mat for example, or a yoga mat, something like that), again while you are present to begin with. He learns to associate the mat with the kong and an enjoyable time, then as with method 1, you start to build up time with you gone, giving him the mat and the kong before you disappear. I like this one the best because it begins to build an association of really good stuff for him happening with you disappearing, so you disappearing begins to become a nice thing rather than a source of stress.
All three methods take time of course - there is no quick fix for this. Looking at how the dogs are arranged can help as I mentioned above, also perhaps looking at what food he's on, when he's fed (like us, dogs tend towards being more settled after a decent meal), walking routine and so on.