I agree with the PP that it really depends on what you want, and your kids' sleeping habits. My two shared a room in our old house, and they loved it. My kids (boy/girl) are 13 mos apart, and my son is a really good sleeper, so even when my daughter woke up A LOT during the night as an infant, he would still usually sleep through. The baby was in our room for about the first 2 mos or so, and then we put them together. My son really enjoyed having his sister there. It helped that I did their bedtime routine at the same time, and everyone had stories/songs together then bed. They don't share a room any longer, but it's more because we have a bigger house with more rooms than because it didn't work out.
Doesn't hurt to try, and I agree that your son won't mind that the room is "girly"!