It's fairly normal for dogs to get upset when their owners leave them. It is not necessarily separation anxiety, at least when we're referring to an actual disorder. That he eventually calms down indicates to me that it's not SA. Check out this video by Kikopup to help acclimate your dog to being left alone:
If he's being very destructive while left alone, you have a couple of options:
1. Pick up things that he likes to destroy or block access to things he likes to destroy. If that does not work, you might also:
2. Confine him to a dog proofed room, whether that be using baby gates or closing doors. That could be a bathroom, laundry room, unfinished basement, whatever you can use. If he's a house eater and/or you don't have a safe room for him to be in (eating sheetrock, ripping up carpet, eating trim work, cabinets, destroying door frames, etc.) you can:
3. Use a crate to prevent him from destroying things entirely. You may want to look up "Crate Games" to help acclimate him to a crate if that's something you choose to do. You could also use an exercise pen, but I'm just assuming this dog is an adult and athletic enough to escape a pen meant for small dogs and puppies, so that might not be an option unless he's unwilling to cross a barrier.
What you choose to do is going to depend on the dog's destructiveness, your home, and your personal preferences.
