I get double turn-river checkraised against relative unknown

[secret.asian.man]

40/80 online

fish just left the table, so Im going to sit out in a few hands. I'm HU with an unknown - I asked a friend about him and he told me he was a very solid 300/600 reg.

I raise sb J9o, he calls. Flop is J74 rainbow, he check calls. Turn is offsuit K, he checkraises, I pause maybe 2 seconds and call. River is a 4, no flush. He checks, I bet, and now he checkraises again! I'm put in this sort of situation so rarely, I feel pretty lost.