Actually, if he was fine when you were gone for a lesser amount of time and this is a new behavior with the longer time then it isn't conventional separation anxiety. A dog with true SA panics whether you are gone for 3 minutes or 8 hours.
There is a little booklet by Patricia McConnell called, "I'll Be Home Soon" that helps you figure out whether your dog has true SA and, if so, how to counter-condition the dog out of it.
In the meantime, I would take seriously the recommendations for upping his overall exercise, both mental and physical. Get up early, do a short, challenging training session and then 30 minutes of really good exercise like running, power walking, ball chasing or a mix of all of that and whatever else you and he like to do together.
Also, do leave him something to do while you're gone like a frozen kong or a treat ball or something exciting that lasts a while.
Ruth & the 4 Legged Rescue Gang
Rafi the malaroo
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Gio & Varda, the krazy kittens
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