Your use of as if is correct here. Both sentences are grammatically proper, but have different meanings, because the conjugations of to know are different.
He looks as if he knew the answer.
Knew is past tense. You would use this statement to describe someone who had an opportunity to provide an answer, but did not do so (or did, but incorrectly). The subject could have provided the answer but didn't, and the opportunity to do so has passed. For example:
John is unhappy, because his team is going to lose the trivia game. Cheryl interrupted him and got the question wrong. He looks as if he knew the answer.
This is disctinct from:
He looks as if he knows the answer.
Knows is present tense. You would use this statement to describe someone who, right now, appears to be able to provide an answer. The subject can currently provide an answer, and it is still possible to do so. For example:
Who should I call on? Rick looks as if he knows the answer. I'll call on him.
Regarding your proposed meanings:
He is pretending as if he knew the answer. He is not as smart as he can solve the problem.
He is a smart enough to know. Many people expect him to know the answer
Neither sentence specifically means either of these. The differnce between the two is when the subject could answer (in the past or present) rather than whether or not they could. However, he looks as if he knows the answer can be used to suggest many people expect him to know the answer, because people generally expect appearances to be accurate. Do note that such reasoning could just as easily apply to he looks as if he knew the answer, meaning that one could just as easily infer that many people expected that he knew the answer.