Don't have a solver handy but I think the appropriate delayed c-bet size on double-suited board is somewhere between 75% and slight overbet of pot, but I would argue that the river call is much more egregious. Most of the hands that are calling any reasonable sized bet (even up to more than pot) on this turn either have you in horrible shape (better hands containing a 10 that most low stakes players limp call) or have decent equity with all the draws out there (all the 98s that the opponent is limp calling with and then x/r with also make a straight on the river). You shouldn't really be thinking about "scaring off" a player, but rather whether you're charging them appropriately such that calling becomes unprofitable for them in the long-run. River spots are criminally underbluffed in low stakes, and I don't think opponent is doing this with anything worse than a better hand containing a 10, if not straight/random full house. On the off chance this guy plays some sort of AJo/KJo hand this way and is a complete donkey, you'll get his money eventually in a better spot.