toddler fights teeth brushing

he was doing so well and then randomly started regressing back to how he was when we first started. its almost to the point of taking two of us to hold him down to get it done. he was almost at the point of standing at the sink to do it himself! we have tried letting him do it, but he wont :( he have a good morning routine and he knows hes going to get it done.

i was wondering, maybe he has a cavity? but hes not sensitive to hot / cold and hasnt shown signs of mouth pain.