Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am so sorry to hear about Zack.
Frankly given your description of his frantic behavior I doubt that Calm Forte for anxiety will be enough to help him. Dogs with his level of anxiety need intensive training, often in conjunction with prescription anti-anxiety medications to allow him to relax enough to learn. None of us learn well when we are very anxious.
If you'd like to try Calm Forte make sure that they have not added the artificial sweetener Xylitol, as many human products have recently begun to add it to mask the taste of bitter herbs and that sweetener is toxic for dogs. The Calm Forte dose for a 60 pound dog ranges from a children's dose to an adult dose depending upon the level of anxiety and the dog's ability to metabolize it. A young, healthy, muscular dog is likely to need an adult human dose because they will metabolize it more quickly but it is easier to start low and increase it so he doesn't get too sleepy.
I don't know his history but if you got him as an adult then he likely has some separation anxiety issues. Many rescues suffer from separation anxiety as they have already lost at least one family. If you've had him since puppyhood and he's always been able to be with you then the issue is that he has never learned to be alone and doesn't have the confidence that you will return. It is terrifying for him to see you leave and for him not to be able to get to you immediately.
Dogs are pack animals, they are socially designed to live with others, be they human or animal. Most dogs would prefer to spend all their time with their very loved family to make sure that they don't disappear.
If he does well in a crate then I would use it while you are gone or cannot be right with him. Many dogs see a crate as a den. It's a secure place that they can be while the family is gone. They have learned that if they go in their crate you always come back. But if they are out in the big house alone they just don't know what to do with themselves, get anxious and get into trouble. If you had a camera on most normal dogs while owners are gone you would see they spend most of their time sleeping. They are pack animals and if their pack members are gone then they rest up waiting for their "pack" to come home and play. So don't feel guilty about giving him a place he can feel secure in and rest in. In time as he realizes you always come home then he may be able to be out in the house but for now if his crate gives him security then let him use it.
If however he is tearing up his crate and is very anxious in it then that's not the way to go.
Ideally while you are working on training him he should only be left for short periods at a time anyway. If you must leave him for longer periods you'll need a friend or dog sitter to come in and check on him, let him out to eliminate and make sure he's OK.
Work on leaving for short periods of time (initially minutes) and always try and make coming and going boring, don't make a huge deal of it as that increases anxiety. Change your routine so that he cannot ramp up his anxiety about your leaving before you've even left. Pick up your car keys and walk around the house. Put on your coat and shoes and walk around the house. Practice him going in and out of his crate and staying there for a few minutes even when you are home. Give him indestructible play things (like a Kong ball stuffed with a treat he has to work to get out) to do while you are gone. Leave a radio or TV on for company, initially both while you are home and away so that the noise level is the same, home or not.
Make sure that you ignore him for the first several minutes when you get home. Busy yourself with mail, or putting things away so that coming and going is never a big deal. Once you've been home for a bit then sit down and give him attention.
Some of the OTC products for separation anxiety are safe and effective, but they must be used in conjunction with training. Medication alone almost always fails. Here are some links that may help you: http://www.bestfriends.org/theanimals/pdfs/dogs/separationanxiety.pdf
As far as over the counter medications I do like DAP products (dog appeasement pheromones) which are synthetic analogs of a calming pheromone a bitch produces while nursing. These come in sprays, collars and diffusors. See this link for information about these products:
See this link for some examples:
I also like products made by Bach's Rescue Remedy: http://www.bachflower.com/Pets.htm
These products can help, but they often are not the full answer. They simply calm him down enough so that he is able to learn. If he is terrified and extremely anxious then he won't learn to comfortably be alone.
If these aren't enough I would discuss medications, such as clonicalm or amitriptyline, with your veterinarian. He is young and they likely won't need to be forever, just until he learns to trust that you are coming back and it is OK for him to be alone in his safe place.
Let me know if you have any further questions.