No, he doesnt "have" to do anything i suppose, but i think he understands the gravity of the situation especially given the fact that its already blatantly obvious the deepstate and media are trying their hardest to take Trump down, i highly doubt Mueller is going to go after Trump on some piss ant move like this, knowing that there will be violent riots from half the country after Hillary was seen to have gotten off the hook. Mueller must understand that really the only way that this whole thing wont severely undermine democracy in the eyes of half the country is if both people get the same treatment.
IMO its not going to happen unless they get the smoking gun, so far all libs have is drip drip drip leading to nothing, if don jr. was caught offering something in return then you might have a point but until there is a smoking gun its nothing, cause without the smoking gun then there is all kinds of behaviour in washington that needs to be examined.
There is either going to be alot of perp walks here or NONE, my bet is on the latter, its the same reason they chose not to prosecute Hillary when it was right on the edge with the emails.