At the risk of being slapped around: Put him on a range.

The straight is an 8-high straight. The only real question here is how likely he is to have played a 9x hand. If a couple of 9x hands are smack dab in the middle of his range there's a slight chance you want to consider folding.

Most of the time even if 9x is in his range so many other hands are also in his range that you can't not call.

How much did you bet out on flop and turn? (Did you give him a good price on a draw?) Did he already have a straight? Did he have a combo draw (with a 9) to a straight that allowed him good odds to keep calling your bets?

More relevant to the calling question than the size of his bet relative to buy-ins - what size is it relative to the pot? If you call and he has a 9 you lose your bet. If you call and he does not have a 9 you win your bet back plus half the pot. If the bet is much larger than the pot this is a situation where he doesn't need very many 9x hands in his range to justify a fold. If his bet is small relative to the pot he cannot possibly have enough 9x hands in his range to justify anything other than a call.

If he makes a pot sized bet and his range (counting all hand combinations) has 1/3 of hands including a 9 it's an even money proposition. If he makes a double pot sized bet and his range (counting all hand combinations) has 1/5 of hands including a 9 it's even money. If he makes a half pot sized bet and his range has 1/2 of hands including a 9 it's even money. If he makes a quarter pot sized bet and his range has 2/3 hands including a 9 it's even money.