Dizziness

My 21-year-old son has dizziness. I think it started during puberty. It only appears when he changes position quickly from lying to standing or from sitting to standing. As soon as he gets up, it gets dark before his eyes and he has to grab onto something and not move. He has severe motion sickness. In the past, he had problems with bladder infections. He also complained that his eyes often watered. He often complained about having little energy, but I think its part of his personality. He was always like that. He is a phlegmatic. Controls emotions well. At the same time, he is also less emotional than most people. He has always been extremely calm and patient. He is stubborn, generally cheerful, ironic, likes to laugh, and is almost always in a good mood. Optimistic and always smiling. He ignores his problems. He doesnt even want to talk to a homeopath because he says he doesnt mind the dizziness, even if it happens several times a day. He is a professional diver and has annual medical examinations, he loves his job and I wouldnt want dizziness to affect his work. He says he doesnt dream at night. As a child, he sleepwalked and talked in his sleep. Not anymore. He rarely gets stressed, but when he does get stressed he bites his nails and sometimes skin on his hands. Sometimes he picks at the skin on heels. He prefers warmth rather than cold, although he tolerates any weather well. He prefers cold meals (he always waits for dinner to be cold) and cold drinks. He has always had an aversion to tobacco smoke. He is slim, even skinny. Hes always had a rather poor appetite. He only eats a few things. He doesnt like fruit and vegetables. He doesnt like coffee. He likes milk and loves sweets.
Is there any medicine that might help him?