You’re not playing Java so I dunno if this will work but I had a similar issue at one point and it was other villagers “claiming” the workstation first, even when they couldn’t path to it. They’d eventually release it because they couldn’t path, but with enough of them, it’d take ages to sort itself.
Try making sure there aren’t any spare lecterns out of his reach, make sure the rest have their workstations, including non-librarians, and if needed throw down a bunch of lecterns yours can reach, so that there’s definitely an extra.
If that doesn’t work or you’ve already got that kind of a situation, try removing him from the rest and bringing him somewhere where he’s the only villager, and a good distance from any workstations at all, so he loses any existing stations he has. Then give him a lectern and if he takes to it, you should be able to move him back after.