There are 52 cards in a deck. Any ten, jack, queen, or king, will give him black jack. Assuming it is a one deck shoe and you have none of those cards in your hand, you are playing one hand, and you are the only one at the table, the odds of him having blackjack are 16/49. This will vary slightly with multi deck shoes and multiple showing hands. However unless your counting cards, insurance is always a sucker bet.