He should know he's benefiting from is essentially large scale fraud
No he isn't, nor did his father. His father used some legal tax avoidance vehicles before Cameron was born. That is not a big deal.
No-one is required to arrange their affairs in order to pay the maximum possible amount of tax.
If governments are stupid enough to make their tax laws so complex that smart lawyers can find loopholes they cannot then blame the people who take advantage of those loopholes. The sensible thing would be to simplify the tax system so that the loopholes were eliminated.
Of course, where there has been illegal activity that is another matter. That should indeed be dealt with appropriately. But I agree with others that, even if there was illegal activity (and none has been alleged with respect to his father) he is not responsible for his father's actions.
if you are the person who is crusading against these tax schemes, legal or otherwise, and you have personally benefitted from this (which we don't know yet because Cameron's saying it's a personal matter) there is a huge conflict of interest
If you are crusading against tax schemes that you may have benefitted from indirectly that is not in any way a conflict of interest.
If you are policing tax schemes from which you directly benefit that is a conflict of interest but there is no suggestion that is the case here.