this is a hard one hun...my dd1 was a bit like this...i found its a phase really, and the more i got her out the more she did it IYKWIM? Im not saying let him scream his head off but maybe some form of gradual withdrawl might be an idea, so you comfort him, put him back, comfort him ,put him back or even get down to his level and try to comfort him WITHOUT getting him out - dont shout over him just be there in front of him holding him in the pram...
If its not a phase it could be some sort of phobia..