Lets assume we have no history with the Villain. I think:

1. based on his 3b cold call and river donk line we can probably say with pretty high confidence that he is a volatile player profile.

2. Texture data point is that, the turn is rainbow.

3. From that we can conclude that only Kx which arrives OTR is AK. This hand might 4b preflop at some frequency, but also fold on the turn sometimes.

4. Continuing with his value region, I think we should expect some 66/44/22, but those hands might either fold pre or take more aggressive line. On this river it seems more like that he wants some folds rather than calls (a little bit of projection by me.. counter argument might be that we are no longer value betting JJ/QQ, so he wants to maximize value with 66/44/22)

5. Lets look at his possible bluffs: well, maybe some AQ/AJ, 33,55,87s.. hard to say, because the bottom of his range is probably Ax which might think it has some showdown value (thats a bit assumptive though, because we are repping pretty strong)

6. The volatility aspect is pretty important here, because there is some non-zero chance that he is just spewing here with 98o..

7. if we werent playing on anonymous site, I think we should also take into a consideration that this player is new and therefore the chance that he is a recreational is even higher.

All in all, I dont think its clear, currently I am not ready to find this (probably exploitative) fold, just because I am not ready to give him/this line enough credit.. but expect to lose maybe like 50-75% of the time