They have mouthwashes for young children that colors their teeth so they can see the 'dirt', reward him when he's able to get all the blue off his teeth, make it a game, you can use the same mouthwash and brush your teeth together. A soft toothbrush is needed for your little guy. You can finish off with a finger tooth brush on your finger to clean his teeth. It's like a rubber tip that has soft rubber bristles on one side.
The necessity of caps may be due to soft or incomplete enamel on his teeth. My brother had teeth like that and he ended up with caps on all his baby teeth, porcelain caps up front and stainless steel caps on his molars. His permanent teeth were just fine. He may have tooth pain because of this anomaly, so using a toothpaste for sensitive teeth may be in order.
Brushing his teeth should be morning and evening events, twice a day. Again, give him behaviors to emulate, like watching you or Dad brush your teeth, do it together. He can't do something he doesn't see others doing.
The general anesthesia observation is heartbreaking, but this is so much easier for him and the dentist. He won't mind the dentist because he won't have pain to associate with the dentist. That was more about you than your son, remember that and don't show your fear when you take him to the dentist. He will pick up on it and fuss for you and him.
It gets better!!! I promise.