Does he get a long walk & exercise in the mornings ? That would help , to tired him before you leave. Also , you can give him a frozen Kong (filled with peanut butter) before you leave. This would keep him occupied for a while. Some people say dogs will act up right after you leave , not so much in the middle of the day , so giving him something to distract him before you leave would help.
you're right about citronella collars , sometimes they work , sometimes they don't. My sister tried it with one of her goldens and it didn't work. You can ask your vet if he has any for rent , that's what my sister did , it was less expensive than to buy one.