Babies and toddlers need comfort to get to sleep. The very easiest and most independent way a baby can get that comfort on their own is through a dummy (sucking is evolutionarily a comforting and soothing mechanism).
When you took the dummy away at 12 months, what replaced that comfort mechanism?
Some children snuggle a blankie or special teddy for comfort. Some self-stroke - they may stroke their own nose or ear or example. Some children twiddle with their hair. Many children use their parents for comfort to get to sleep- either by cuddling or rocking, or just the reassuring parental presence.
There are lots of different things children do for comfort, but the point is that it takes until around about pre-school age (3-5y) until they don't need comfort and can do the more adult mechanism of Tired>Close eyes>Sleep without needing to feel comforted.
It sounds to me like when you removed the dummy, you didn't establish an alternate comfort mechanism adequately enough.
So the first thing I'd suggest is to stop beating yourself up that your child can't explain what's wrong. Of course he can't, because he's not an adult and doesn't understand what it means to not need comfort at bedtime. He's not going to be able to verbalize that. Just accept he does need comfort every time he goes to sleep.
Next, help him develop ways of achieving that comforted feeling. Long term it would be best if that's a comfort mechanism that doesn't require an adult. But initially it will probably also need adult comfort while he learns how to relax into sleep.
Maybe introduce an extra special snuggle toy. Snuggle with him and the toy at sleep time. Then he learns to associate the comfort of the toy with comfort from mummy too.
Try to be patient and take your time. But like learning all new skills, learning to self comfort takes time.