If you have a 6 month lease that is silent as to overnight guests the landlord cannot terminate the lease early by givng you an addendum you do not sign or by giving 30 days notice.
He cannot evict you without filing an unlawful detainer complaint, serving it on you, and having a trial in approximately 30 days after service.
You can have your boyfriend over as often as you want. If landlord does not like it, he can agree to release you from lease without any liability to pay rent after you leave. Get it in writing.