Hi, I'm new to the forum

We just adopted a 13 month old fox hound/boxer mix, Sammy (so we think - he might be other breeds as well) and he has severe separation anxiety. Right now my husband works from home, but in 6 weeks will be starting college again and away for the day. Right now he is only alone for a little while when we run errands or something. Sammy is perfect while we are home - housebroken, likes to play but is content to lounging around with us as well. Learning tricks and obedience very well. When we leave however, he barks, whines, howls, and would get into things. So, we're trying crate training. We don't much of his past except that a family took him in as a stray, had him for 4 or 5 months then turned him in to the humane society where we adopted him after he was there about 10 days.
It started out okay, letting him get used to the crate - he even layed in it on his own for a few minutes. Then came the time after a few days to close the door while we were home. Well, he thought we were mad at him and sulked the rest of the day and now will have nothing to do with the crate. He goes in OK when we have to trick him with treats but always leaves one paw outside the crate, so when we leave we have to get that one paw inside. All the chew toys and stuffed animals in his crate doesn't help.
We had started with a nylon foldable crate because we travel a lot to visit family, sometimes overnight, but he figured out how to unzip that one - and once he figured out he could get out, leaving him in there meant he was determined to get out again. So we bought a metal crate.
Does anyone have any advice on getting our wonderful dog used to his crate again? he still goes in for treats, but warily, as if he expects to get locked in. We'd appreciate it!!