I think Phred's suggestion is a good one, seeing if you can find out how long he howls or fusses after you leave. If it's only a few minutes then it probably isn't too serious except that I know it must break your heart, even if he only does it for a few minutes. I know you have other issues with him and have talked to a trainer so if you feel looking into a medication would be something that might be good for him then I'm sure your vet would let you know the options and possible side effects, etc.

Good luck, I know you love Lefty with all your heart and obviously vice versa!