Louis Simpson has not been seen since a blaze broke out at Fairhurst Hall in Chorley Road, Parbold in Lancashire, at about 15:00 GMT on Sunday.
Police said he was with three boys who have all been found. Firefighters believe Louis "may be inside the barn".
His family have been at the scene while fire crews and police search the area. Police are appealing for information.
Det Supt Neil Ashton said it appeared the group of friends were "playing in a rural setting" and were at the barn when the fire started.
It is not yet known how the blaze started. An investigation into the cause is under way.
Members of Louis' family are doing what they can to help with a search of the surrounding countryside.
Firefighters are damping down the barn, which had been storing about 250 tons of straw.
A 14-year-old boy who was "playing" with friends is missing after a fire at a barn that was storing straw.