If you want to keep your apartment, you'll have to hire a lawyer (unless the landlord is bluffing and hopes you'll just leave). This is a difficult kind of case, though the landlord has to prove that he actually will use it for his son. If he can prove that, you probably won't be able to keep your apartment.
Still, there are some variables: the judge who hears the case, your lawyer, how hard and long you're willing to fight the landlord in court (in which case he might offer you a settlement to get you out quicker than he otherwise will be able to). That unfortunately can also get expensive for you.
If you're paying at or near market rents, it may not be worth fighting, particularly if you believe the landlord really intends to put his son in your apartment. If your rent is low, the fight may be worthwhile.
This is not expert advice. Check with tenant organizations like Met Council, etc. for another point of view and for attorney recommendations.
<small>[ December 20, 2002, 02:37 PM: Message edited by: mkraft ]</small>