I'm so sorry to hear this. He does sound like he's too young for dementia unless he's older than you think he is. I know that before my RB Storm had to be put to sleep, he would also some times start howling like he was lost. It would only last a minute or so until he heard my voice and I helped to calm him down but it was scary to watch. His back legs would also give out on him and he became so bad that he could barely walk and he also had a hard time using the litter boxes. His health was deteriorating before my eyes and I knew it was time to let him go. Hopefully you'll be able to find out what's causing this. Maybe it's some kind of a neurological problem. Do you have a vet that specializes in cats near you? Maybe you should get a second opinion. Lots of prayers and positive thoughts are being sent his way.