Why wont my 7 wk old son sleep more then 20 mins at a time.?

Since my son was born he wont sleep more then 20 mins at one time day or night. I have called and spoke to the Dr. but she dont know. I have tried everything, the bouncer, car seat, elevating his bed, buying a binett, playyard, different crib, a bath before bed,mage,even sleeping on my chest nothing is working I also have a 18month son as well. I am very exhausted and very moody these days. I aslo have no support other then my hubby. Someone please help me. I really dont know what to do next.