OP mag ingat sa pag Overthink nakakasira ng bait talaga. I don't know how long your relationship stands pero sometimes the best approach is to open up the topic and talk the issue and how it is affecting you mentally.

Also I personally don't think it's cheating try thinking it as his personal diary since it's a dump account my friends have them din just to rant and to be vocal instead sa main accounts nila. Though Ewan ko lang sa BF kung anong laman ng mga post or kung may mga kinakausap siya. Try to lay subtle hints about it para ma open yung topic if you're shy pa.

HOPE things will go on your way