First, don't give up on your boy. As Jackie said, he's been 'in prison' for y his entire life and that five years would seem like 500 to us!
Also re-read Jackie's post she's given you fantastic advice and has had wonderful success with very reactive dogs - reactive can be anything untoward not just aggression.
This is an 'unusual' way to get a terrified dog to accept a leash, yes you do have to 'think laterally' at times, but it did work with a friend of mine who came and ask advice about her rescue dog.
What I did, when all else seemed to be failing, was to 'wear' the leash around my neck - like a scarf - and just walk around, have a cuppa, chat etc. sitting on the floor. O.K. so it looked weird, new type of accessory? But, after a while her dog got very curious about this 'thing' around my neck. I let the dog check it out, sniff etc. without making any real 'fuss'. It would have smelt strongly of dog as it was one of my dogs leads. Off trotted the dog, then back again. I rinsed and repeated this several times over the next few days. Then, when the dog had almost lost interest, i.e. wasn't running away in a blue fit, took it from my neck and just laid it on the floor. Again, same thing, the dog came up sniffed, backed away, came back etc. etc. It was then that I just GENTLY dropped a treat by the lead. Again rinse and repeat. In the end her dog got to 'know' the leash and we proceeded as Jackie has described so well in her post.
With crates, I don't use them, but I would say one thing PLEASE NEVER PUT YOUR BOY INTO A CRATE WHEN HE'S DONE SOMETHING WRONG!!!!!!!!! Crates and mats are 'quiet areas' not a punishment area for a dog.
And with your boy that crate may just bear a bit too big a resemblance to the 'prison' he's been in for five very long years. Mutts advice is excellent on the crate.
Get him some good chew toys, Deer Antlers are fantastic. My young puppy has them, took her a few sniffs and she ignored it for a while, but now LOL put it up high and she'll get it down and chew away peaceful for quite some time. She's actually right here now chewing on it. Kong toys are also terrific, and maybe even a soft toy, some dogs love them.
Please take it very, very slowly, almost at full stop speed with this boy. He's absolutely terrified of this whole new big world, and needs very patient, gentle handling, don't rush anything, it may take a long time and then again, with your love and help, he may suddenly make a quantum leap forward.
Jackie has said to use your other dog as a teacher and this is really excellent advice. My new puppy was very timid, having been rescued from an 'abusive environment'. She was placed in a foster home, while all the legal stuff was gone through, and then came to me. Her foster Mom (who's a real sweetie) worked purely on socializing Leaf, step number one of course. So when I got Leaf she new nothing about lead walking (I was warned). My older boy (who's 13 years young) is excellent at loose lead walking, heeling etc. So I used him to 'teach' Leaf. On her second walk (she wasn't afraid to leads and collars so please note that) she was walking along as if she'd been doing it all her life! So definitely use your older dog to help this newbie learn 'the ropes'. Dogs themselves are amazing teachers! I've seen it happen time and time again.
Good luck pack a heap of patience and remember one baby step, even just half a step, at a time. Not too much either, one area which is the most important first, then onto the next. You need to gain his confidence and trust before he'll really 'get' anything. Once you do, I'm with Jackie, my bet's on the dog!