If he was raised in a hoarding situation, he probably does miss other cats. Cats usually only become actively anti-social with other cats when they are raised as only cats or had to live on the street for awhile - where all other animals are competitors instead of company. One of my cats is like that - she is only intermittently tolerant of my other pets. However, since you say you're in a studio apartment, I wouldn't advise another cat.
Since he's standing on your bed yowling - he probably wants your attention. Interact with him and see if you can get him to get comfortable enough to allow you to scratch behind his ears until he flops over - after that, he should fall asleep (on your hand, of course, because he's a cat.)