How did you "explain" divorce to your preschooler?

supermom2008

My son is 3.5. STBXH is still living with us but is moving out next weekend. Originally, he was planning on moving in with his parents, so we told DS that "Daddy will be living with Grandma and Grandpa". DS was happy because he loves going to his grandparents. But now, a change of plans, and STBXH is getting an apartment, pretty close to our marital home. We  started telling him that Mommy and Daddy are getting our own houses, that he will have a room here and a room there, and plenty of toys to play with and have fun. He asked why...and his Dad jumped in and started talking about how he'll have two room now instead of one and how much fun it will be. I feel like he needs a better explanation but don't know what to say.