If he's on a straight or a flush draw, your bet could be 300 and he would have to call that with the pot the size it is now. 350, even.
Probably even more, if he factors in implied odds.
If you're right about him having the nut flush, then it doesn't matter how much you bet, of course. He will happily call an all-in here or on the river.
If he has a set, you want his money in the pot now, before a fourth flush card hits the board, at which point he would be a whole lot less inclined to call if he doesn't have a high card of that suit himself.
If he's on a draw, 350 is about right to give him odds to continue, since that was your specific question, though.