We were unable to crate train our severely anxious dog because she had barrier anxiety as well and would never calm down in the crate- she would slam herself against the grate and chew until she could get out. We worked with a behaviorist and once we was housebroken we stopped the crate and did other exercises- like continuously entering/exiting, wearing our outdoor clothes and sitting on the couch, etc

You might work with a CSAT and behaviorist. I did a desensitization program with both.

I like to give my dog something that they CAN do. When my dog is going to be alone for a full day, I leave him with a frozen peanut butter Kong. I also remove (as much as possible) things he might be tempted to chew that I don't want him to. I want to set him up for success as much as possible.

You first need to verify your diagnosis - cameras set up to watch his behaviour when you leave. At 8 months, what your describe is within the realm of normal adolescent exploration. Once you work out if it's true separation anxiety or boredom, then you can make a plan. IMO true separation anxiety dogs usually require psychotropic medication and behaviour modification; you'll need a veterinary behaviourist and trainer for this. Get some footage and verify the premise of separation anxiety first.

I crate trained my pittie at two years. It’s not too late but may not be the real answer for separation anxiety

Not a trainer but we crate trained our 1 year old dog and it went fine— start on a weekend or other time when you’re around so time builds up a bit (I.e once you get to closing the door and leaving it’s only a few minutes and then a few hours). Also music on or some other stimulation can help. ❤️

There are a lot of decent recommendations here. Something I would add is not to ignore the help that medication for anxiety can provide. Dogs (and humans) can’t learn when they are over a panic threshold. So until you can get him to a point where he isn’t over that threshold, he doesn’t retain the recognition that he did not in fact die when he was left alone, and can’t improve.

Is it truly separation anxiety or is he just a mischievous 8mo puppy? Definitely start kennel training ASAP and consider adding regular obedience training to help round out his overall behavior.

It’s gonna be harder to crate train now then it would have been as a new puppy but it’s still worth doing, especially bc he’s still pretty young.

Get one of those treat balls, fill up as you get ready to leave, give to dog as you do, they'll never even know you left