Quote vickidink: Think about it for a minute - He knows that as soon as his bone marrow is drawn and saved for Danny, Sonny will have him killed. Julian plans on sticking around and to be available for Danny if Danny needs bone marrow in the future. Julian hasn't said he will never donate for Danny's sake. If Julian wants to protect his life and still ensure that Danny will be looked out for in the future should he have a relapse, Julian should have the bone marrow saved now and not let anyone but Sam and Silas know it (Alexis tends to run at the mouth when in a stressful situation so I wouldn't trust her with any secrets). Then Julian has everything covered - Danny's future and Julian's life spared from Sonny.
Frozen bone marrow is only viable for seven years, and depending on what type of treatment you need it may only have viability for 6 months. So not much of a plan, and I doubt Sonny would have done it if the tables were reversed. Sonny is still playing playground games, he isn't prepared for the adult games Julian can play. I'm really enjoying the cat and mouse between Julian and Sonny as Julian one ups him at every turn.