They fed him honey and water, it is probably just enough to survive though Bran would have lost a lot of weight.
"Will Bran get better, Uncle?" little Myrcella asked. She had all of her mother's beauty, and none of her nature.
"His back is broken, little one," Tyrion told her. "The fall shattered his legs as well. They keep him alive with honey and water, or he would starve to death. Perhaps, if he wakes, he will be able to eat real food, but he will never walk again."
A Game of Thrones, Tyrion I
It’s worth noting though that his connection to Summer seems to have helped his recovery.
"The gods alone know," Tyrion told her. "The maester only hopes." He chewed some more bread. "I would swear that wolf of his is keeping the boy alive. The creature is outside his window day and night, howling. Every time they chase it away, it returns. The maester said they closed the window once, to shut out the noise, and Bran seemed to weaken. When they opened it again, his heart beat stronger."