The spec says that a node should refuse a block if the timestamp is more than 2 hours into the future. If the timestamp is too far into the past, I'm guessing that most nodes will also reject it (since it would appear to be older than a block already accepted as valid).
That would be true if he is mining solo.
If he is part of a pool, I do not believe it is required as the pool is generating the timestamps he would be hashing.