Restless sleeper, sleepless mama!
I feel for you! My dd is about 18 months and is still wakes multiple times during the night. Her molars are also coming in, so some nights are very rough.
I had a couple of follow up questions: does he nap during the day? Have there been any other periods of time when he did sleep better? Are there any signs of possible discomfort (teeth, belly, body parts)? Does he seem to wake up due to internal stimulation or something in the environment?
You have other children, so you are more experienced than myself. The purpose of my questions are to get a more complete picture. Hopefully the collective experiences of members here will be of help!
Try feeding him foods that promote sleep right before bed, if you haven't already. Easy to google there has to be a physiological problem as to why he cant sleep. Change drs if they aren't helping or at least ask for one of the other drs in the clinic for help. Blood test might be in order. I'm surprised Drs haven't figured it out by now. There has to be studies why some children sleep beautifully and others struggle with it.