"He looked indifferent" is grammatically correct. The second sentence could be rewritten as "He had an indifferent appearance" to make it grammatically correct. But note that both sentences describe are describing the subject's appearance. The word "indifferent" describes someone's state of mind - an indifferent person is someone who doesn't care. Saying "He looked indifferent" has a slightly different connotation to "He was indifferent". In the first sentence, the subject is not necessarily indifferent - he might care deeply about his job even though he appears to be indifferent.