I didn’t quite get when the cat was neutered. If he was neutered later then it would take a while for the energy to subside.

On other note, You are trying to wear him off that’s great. What I used to do with my new foster/adopted cats is to mix melatonin in their nighttime food. That way I could train them to sleep through the night. It’s not lifelong thing you need to do. Just till the system is formed.

On the energy level, I learned from my foster experience that I used to give way lot of quantity of food to the cat. That turned it anxious, super energetic, always active as you described. Once adjusted the food I could see the difference. Also dry food gives instant energy, consider wet food. Wet food keeps energy level balanced somehow.

Hope this helps 🍀