Unfortunately that's what some Dexters do! They can be notoriously noisy.
I have a Dexter cow that I put in our back field for a few weeks (not on her own) and she was constantly bellowing. Some people then give them food to shut them up and that makes them worse as they then start bellowing as soon as they see you. With Poppy, my cow, I made a habit of going down to the field several times and not feeding her so she wouldn't expect anything. In the end, I threw a bucket of water over her to shut her up. The neighbours never actually complained but I wouldn't have blamed them. She did eventually keep quiet when she realised she wasn't getting any treats. But in winter, when she's inside, she starts up again whenever anyone appears. That doesn't matter too much as there's no houses nearby.
I don't think you'll ever cure him of it, especially as his previous owners had the same problem. (Bet they celebrated when he went!)
However, to kill him if he isn't fat will be a waste of meat. I got a dexter bull slaughtered before he was fat and the steaks and quality cuts were tough as there was no fat around to make them succulent. The mince was ok. I should get an experienced opinion as to whether he is fit for slaughter or not.
If he needs further fattening then I'd go with Sally's suggestion of maybe getting another quiet animal to go with them, or find a remote field.