My guess is that it's not just separation anxiety or, like others said, other behaviors would be exhibited as well. Whining, howling, scratching at the door when left, etc. In my experience separation anxiety doesn't suddenly crop in dogs without some sort of trigger as well- moving, change in the people/animals of the household, sticking the dog in a crate without training, change in routine (dog is left alone a lot more because someone went back to work) stuff like that.
To me it sounds like a young, very active dog entering the dreaded teenage phase. We went through it with our Brittany (the spaniel part is no longer considered part of the correct name, fyi) when I was younger. With proper management and maturity they do eventually grow out of it. I'd suggest going back to crating when you're gone. If you haven't used the crate in awhile do be aware that your dog may have taken several steps backwards in his crate training. Start from scratch there if you need to. It's really important to make sure that you keep him away from inappropriate items while you're away, so that's why I recommend revisiting the crate for awhile. An alternative would be to dog proof a room in your house, such as a bathroom. Remove everything that the dog can destroy and instead put appropriate toys in there. We used the garage with our Brittany until he was at least a couple of years old.
Leave him with appropriate toys to chew if you think it's safe. Our Brittany had jaws of steel as a puppy so we wouldn't trust him being left on his own with chews, but I leave my current rat terriers (lighter chewers) with appropriate chews when I'm gone. If you catch him chewing on an inappropriate item when you're home then redirect him to a more appropriate toy. If you see him chewing on his own chews then praise him heavily. Make sure he knows that chewing on his toys and chews is a GOOD thing and it makes you HAPPY! Too many people neglect to reward that kind of good behavior, and it makes it harder for the dog to grasp what's ok to play with and what's not.
Also make sure that the dog is getting plenty of mental and physical exercise. Sporting breeds are very active dogs bred to run all day long. At his age he's still a bit too young for on leash runs, but make sure he has lots of opportunities to run at liberty if at all possible, take him for lots of fun walks, play ball with him, play dates with other dogs, etc. A tired dog is a good dog, but don't overdo the hard exercise as his joints are still developing. Also make sure you're doing daily training sessions with him to challenge his mind. I'd recommend doing something active with him and a short training session with him before you have to leave for a fair period of time. A bored and understimulated dog will create their own activity, and sometimes that activity is tearing up your stuff.
