He hasn't "forgotten" how to be crated. He has had a taste of "no crate" and does not want to be confined again, thank you very much! In hindsight, it would have been good to retain some of the crating routine, even when someone was available to be home with him, even if it meant inventing reasons to go out. I find that a dog who loathes being crated dislikes it even more after he has had a period of not being confined. So, what to do? First up, I agree, teach him to go in by himself. If you clicker train, that should be easy. (if you don't, learn!). This is how I would proceed. Don't lure him or try to force in any way. Just you, the crate, and him in a room that he can't leave. Click and reward whenever he ventures near the crate. Keep rewarding successive approximations until he is close, then closer, then feet in, then all the way in. Do not be anywhere close to him! Click and let him come to you for the treat. That resets him for the next attempt. When he does go in, do not go near the crate yourself or close the door. You need to work with this until he goes right in, then wait to click until he is there just a little longer. Work on this until it is solid! Then name it. Once he understands and will go in on command, you work very slowly at being closer to the crate yourself, (try placing yourself at different spots in the room at first, not necessarily near the crate, to give your command), to rewarding him when he is in the crate. Don't rush to close the door. Eventually close the door momentarily, then let him out again. Yes, it may be a slow process…...but you never know. My guy, who does not like to be crated, learned to "kennel" in about 15 - 20 minutes! (if you aren't making good progress, quit when you have something to reward......close to the crate, for example......and start again another time when you are both fresh).
Once you have that down, a good strategy when you are going out is to put his treat into the crate and lock the door so that he can't have it. Do this half an hour before you leave, to give him a chance to build up a craving. When you open the door, hopefully he will want to go right in. If not, evoke the command, close the door after him, and leave immediately. At this point, a video to let you know what is happening would be useful! Starting with a short absence, then increasing it gradually, is usually recommended but may not be the best strategy with all dogs. A video will tell you whether he settles down quickly and is quiet, in which case staying away your usual length of time is a better option, IMO.
Of course, it's possible he just won't adjust to being crated again. My guy, who had been crated every day for three years before I got him, will have nothing to do with crates now. Yes, he will go in on command, but if I close the door and leave him it is freakout time. However, he is excellent loose in the house, so I have no need to push the point. His previous crate experience was always with other dogs crated beside him, which he could apparently tolerate. Being crated alone was another matter, and after we struggled with it for a short time (during which he only got worse), we gave it up and have been very lucky, because he is the best Basenji I have ever owned in terms of not being destructive.
Good luck with your boy. If the crate is a no go, I agree that using a larger pen of some sort (or a room?) might be the solution.