Paint his wall that his bed will be on blue, and then leave her wall pink unless she wants it changed. Then do the other walls neutral or in a chalkboard paint. Or you could paint the whole room a neutral color and then do different colored bright painted picture frames (like hot pink, purple, lime green, turquoise blue, etc) but inside the frames to the chalkboard paint. That's what we are doing in our kids rooms as we just moved into our house and i don't want crazy colors on the walls lol
Look online at split gender themed rooms :) there's some pretty cool things you can do
You could just repaint the whole room and do a neautral theme, or leave it pink for a while (he wont care) or do half and half like you said. In the end whatever makes you happiest. Our kids room is green with a mix of fairies and cars/ trucks.
Could you re-paint the whole room a gender neutral color like yellow or green?
Maybe do something neat for both genders like paint the ceiling as a sky and the walls as a forest?