Broker's Fee - Broker is the son of the Owner

Unless your apt is rent stabilized, or you have some contractual renewal right, you have no right to renew the lease at all. or at any price. Whether the owner's son has some financial interest in that decision is beside the point.

Yes it is legal if you are a market renter. And if you choose not to renew your lease because it is too high the landlord has every right to show the apartment to new prospective tenants. The fact that his son is the broker is irrelevant.

What neighborhood is this in?

You shouldn't deal with people like this. Yes everything here is legal, you are entitled to nothing; however you owe it to yourself to do business with a higher quality of person.

Is the son involved at all in the management of the building?