Carry on with the positive associations with the crate and building up the time he's left slowly, but I would advise caution on getting him used to long-lasting treats to distract him from your absence.
I say this as someone that made the same mistake and now has a great lanky wuss of a Lurcher that developed separation anxiety.
If you give him long lasting treats/kongs etc, you could end up in the situation I did, where he would be quiet as long as the treats lasted, then start the wailing (or screaming in my dog's case) as soon as he was done scoffing.
I inadvertently made the situation worse, because when he looked up from finishing his treats there was a sudden realisation that I'd gone. Whereas, if I'd taught him to self-settle and be relaxed with me coming and going I wouldn't have needed the distraction in the first place. Treat toys, kongs etc should be used as tools to keep dogs busy and prevent boredom when they have to be alone, not crutches to distract them from their solitude, iyswim.
Having done a lot of research on this, I now know the most important thing to teach is self-settling. You need to work on him doing this with you there to begin with (Google 'training a dog to settle', Victoria Stillwell has a page on it, as do other trainers, including Kikopup/Dogmantics). Then you gradually build up to him settling with you out of sight and for longer and longer periods. If you can set somewhere up so you can build up to being on the other side of a see-through barrier, such as a baby gate that can really help the process.
That isn't to say you can't use stuffed kongs or similar to help him learn to settle. A loosely stuffed kong can be a good way to introducing self-settling if your pup has a particularly short attention span - so you'd do the basic steps to get him onto his mat and not moving, then reward him with the kong to get him to stay there a bit longer - all while you are still within sight at first.
He will most likely pick it up really quickly if you follow the steps properly and you should end up with a pup that has learned to be completely calm when you're not in the same room as him.