Where would you take your kid if they have been very ill for months and you can't get a diagnosis?

My 15 year old son has been so ill and disabled for about 8 months now ever since a 15 minutes laser surgery on his arm last year where he was put to sleep with general anesthesia. He is in constant pain across his chest and just feels very sick 24/7 since this surgery and I can't get a real answer or help. I do know now that they do see some deep malformations in his chest and these type of malformations are very painful, but he has always had these all over his body and I know now that he has P.O.T.S. and I diagnosed this and got it confirmed with a cardiologist. I saw his heart monitor in the hospital about a week after surgery go from about 60 to almost 200 upon standing and the doctor that I told in the hospital said he was probably dehydrated, which I knew wasn't the case he had been drinking loads of gatorade and water, so I revisited this scenario a few months later at a cardiologist office and they reconfirmed that this is one problem that is going on, but even with these 2 problems that I found it still seems like he's so sick that something else has to be the problem also. Especially since this all started immediately after surgery. Can anyone guide me on where to take my son? He has almost zero quality of life now, he's in so much pain and can hardly walk more than a several steps around the house or he's about to pass out and the chest pain gets worse and worse as he walks. His heart has checked out fine, so it's nothing to do with the structure of his heart, just the heartrate problem of going up extremely high as he stands.