First of all I would like to thank you for making this series.
So what do I think he has? For now, lets put on a side that he is tight player. Player on your right opened preflop, and got you and a SB as a caller. BB actually had to pay just 0.2$ for a 1$ pot atm (with his call total pot is 1,2$), in this case he could call with any two. Its a possability, but I doubt that a tight player would do this. Well wouldnt be surprised at these limits, everything is possible
Donk bet on a flop --> probably he was thinking that with it he will push at least two players out of the pot if not all, which he did. Just you called him. The turn paired the board, and he checked, you bet, he min-raises. I'm quite sure he hasn't got pocket fives, and that for sure he wouldn't donk on the flop with lets say A5s or A5o. Does he has pocket nines? Could be, maybe because of that he made that donk bet as he was happy to see trips (he can be quite sure that any Q will pay him off) and also to "protect" his hand against flush draw. IMO players on this limit would for sure call if they would have flush draw!
What is his hand? As said 99 is possible, but IMO he has a strong Q, AQ or KQ. He probably min-raised you as he is "putting" you on a flush draw and thinks that you will fold after his raise. I would suggest push, as most probably you are ahead (in case he hasnt got pocket nines or 5 (less likely)). If you are behind you still have well (in case he has nines full with fives) two more outs, good luck