Wow, lots of good help here!
And such a handsome, sweet looking puppy! He's got that "but I didn't mean to make you mad" look on his face.
We've had Denna since she was 7 weeks old, did everything right (as much as we know how to, anyway), and she STILL feels a need to destroy things... when we're home or away... it's just fun and entertaining for her. Her humans are too boring. I know I've been ignoring her, sitting at my desk too long (I work at home), when I hear her throwing another potted plant around the yard outside... :\ That's when we leash up and go for a good walk - which is good for both of us.
You can't punish the bad behavior after the fact... the dog won't understand, and confusion leads to frustration, which leads to more destruction. You have to catch them IN the act to be able to let them know its unacceptable, and even then, a firm "NO" and redirecting to an appropriate behavior (i.e. sit, down, or 'here chew on this instead') should be all you need. Mastiffs are often very sensitive to their person's displeasure.
When we're gone, Denna has a BIG caste (54" - more than enough to stand up and turn around in - your's looks good for now, too) to stay safe in (which keeps our stuff safe, too). She knows we're leaving the house when I start filling her kong with treats and peanut butter. She only gets filled kongs in her crate - and she'll run to get in when I have one in my hand.
Find something Harley loves (cheese, hot dogs, peanut butter kongs, bully sticks, etc.) and he can ONLY have that type of treat in the crate. You don't have to shut the door on him at first - just get him to go inside, get the treat, and come out.
When he voluntarily goes into the crate to get the treat, then ask him to stay in the crate while he finishes eating it... door still open... you standing outside the crate just to block him from coming out.
When he's comfortable staying inside to eat his treat with the door open... start shutting the door on him when he eats his treat - but open it BEFORE he get's anxious about it being closed.
Start to ask for more time in the crate - maybe sit outside the closed door, read a book for a few minutes - something quiet and non-interactive, just being there to let him know you want him to stay in and be quiet, and it's not punishment (isolation from pack), it's just a place you want him to be. If he lays his head down and is quiet, tell him quietly and calmly how "good" he is for doing it. Maybe give him another treat.
Then you can start moving away from the crate when he's eating his treat - this is where filled kongs come in... it can take a LONG time for the dog to finish getting all the treats out of the kong. The best part is if you can leave and the dog doesn't even realize you've left the room... because he's still working on the treat.
It can be a long process - especially for a dog that might have had a bad experience in the past. Feeding him in the crate is good - that should help the whole process.
For bedtime - we had the crate in the bedroom for a while (when we used a smaller one... the 54" won't fit anywhere but the living room). Now Denna has her own bed on the bedroom floor, and we baby gate the door so she has to stay in the room with us. Half the time (or more) she ends up on the bed with us... but we're ok with that. She'll get off if we ask her to - and sometimes we do.
Overall - I'd say more exercise - and more mental exercise - would be good for Harley. Try doing 10 minutes a day of obedience, commands and tricks... something new each week. Have him learn to "Hold" a treat on his nose and then catch it when you say "Break" (or "OK", or "Catch it")... if the treat hits the floor, he can't have it... he has to catch it! This teaches focus and patience, both... along with eye-nose-mouth coordination (not sure what good that is... but, you never know).
And, have FUN! I'm sure the boys are loving having him around, too. Just be sure they can all be calm together as well as fun and excited together. Napping and reading books together is almost as good as walking together for pack bonding.