Generally, the best thing to do would be to wait as long as possible... until the crib just won't work anymore. BUT, with a new baby needing that crib, that changes things. In order to keep your son from resenting the baby or feeling like the baby took his bed, it's best to do it 4-6 months in advance so he doesn't view the crib as his when you put baby into it. If you plan to put baby straight into the crib after birth, then I'd say, now is the time. If you plan to have baby in a basinette in your room for the first 4-5 months, then you can wait longer. In general, you don't want too many things to change at once for your older one. Trying to get him to move to a big boy bed (which is a HUGE change for him) too close to the time when he has to adjust to a new baby and sharing mommy would be a recipe for disaster. I'd say, now is the time.