Actually you would be surprised at how much better he will sleep when he feels relaxed and secure....and if that is in your room for the time being,why not? He's still just a little kid that still needs you for emotional support day and night as he matures and develops. You don't have to follow the old fashioned advise or do what 'mainstream America' does and battle and fight with him about sleeping away from you. He knows what feels right and it's natural to want to be near others that you love,even at night. He is showing signs of being a very affectionate person, don't 'break' that....it's not a 'bad habit' as many will call it, it is a very natural and comforting way to sleep that fulfills his emotional needs and provides security right now when he still needs it. Don't worry, he won't be in your room at 18!!!
Why not give him a small bed or just a toddler or crib mattress next to your bed if you are really opposed to him being in bed with you? He may adjust and settle in with that since he'll know your so close. And don't worry about the baby waking him, I have a 4 yr old that has slept right through a younger sibling crying for the last 17mo....they get used to it and tune it out. He will still probably sleep sounder knowing everyone is close to him.
I think that him sleeping in your bed or at least in your room will be very comforting to him during the HUGE new changes that are about to take place with the addition of a sibling. That is a very stressful time for an older sibling and will be the perfect way to give him affection, comfort and security that he is already begging for.... Enjoy these times with him. They grow sooooo fast and once the new baby is here your time with him is even more limited so keep him close :-) Best wishes!