Well done for getting the Portpotti! We love it and use if often in our playroom at the Intensive program.
Because the flushing, etc can be distracting for some children, and they can also be really sensitive to the chemicals that you are supposed to use in it, we do not keep water or chemicals in ours at all and that way, there is realy nothing to flush. If the child pees or poops in it then we open the drawer at the bottom so that the flap opens and the pee or poop falls down into the compartment where it's supposed to go and then we clean it out at the end of the day.
If he is actually using the potty to pee and poop in then I would probably let him flush it (if it didn't have the water and chemicals in it to keep flushing), the "dry flush" experience will probably not be that exciting for him, but if he continues to do it I would treat it like an ism and join him pretending to flush your own toilet. If he is getting into the bottom part where the pee and poop goes then it's a health and safety issue so we would set a boundary around it. The way to do that is first to ask him not to, if he continues, sweetly explain you want to keep him safe and then stand in front of it so he can't get to it. You could put the lid down and sit on it with your feet in front of the drawer. If you are intending to keep the water in it then I would also set a boundary around him flushing it because it will use up all the excess water and there won't be any left for when you need it.
All the best!