I would 100% continue showing it and it’s surprisingly very common. I have even seen people accept two offers at the same time and the first one to come back with an executed contract gets the deal (which I think is a little much though). There is no guarantee this buyer will actually sign the deal, so I think it’s completely reasonable for you to keep showing it and let the buyer know it (maybe it will motivate him to sign).
That being said, I also think it’s a little crazy that he wants to wait for the financials. 2018 financials typically come out this time of year and he should be able to get a good idea of the building’s finances based on the 2017 financials. If the building’s finances for 2018 are drastically different, then you can put a clause in the contract allowing for him to get his down payment back. If it is a drastic difference then the lender might decline to lend in the building anyway, so he won’t have an issue getting his down payment back. This makes me think that the buyer may be getting cold feet, which is even more reason why you should continue showing the apartment.
Regarding your broker, is he part of REBNY? If so, it would be against their code of conduct and that of his company for him not to provide you with all offers that are made. He can advise you not to take an offer, but he is required to present them to you. If he doesn't he is not acting in your best interests, which is his job. I would also be a little wary of using a broker that is not willing to continue showing the apartment at this stage. Deals fall apart during due diligence all the time and continually showing the apartment is pretty common place even if it's just to accept back up offers.