4yo son's favorite playmate is leaving daycare - what do I do?

My four-year-old son attends day care three days a week (with his not-quite-two-year-old sister). His best friend there, Timmy, is leaving daycare at the end of the month, and I'm worried about how he's going to take it. I'm looking for suggestions and reports of similar experiences.My son is an outgoing, gregarious kid who thrives in social situations. He makes new friends pretty easily when we go to the park, and he likes to talk to people at the grocery store. However, he also gets attached pretty easily, and he has a sensitive streak that takes rejection hard.He has cousins that he plays with regularly, one of whom he considers his “best friend.” But whenever we talk about his time at daycare, he always starts with stories of what he and Timmy did that day. When he’s pretending or telling made-up stories or talking about where he wants to go (usually Legoland), he always includes Timmy in the story.The daycare operators have been talking about Timmy’s departure with the children, mentioning it frequently and making sure everyone is aware of what’s happening. But my son is pretty young, and I don’t think he grasps the concept of losing someone he knows forever. The couple who run the daycare have spoken to me and my wife about Timmy’s leaving so that we can help prepare him for it. The problem is, I’m not sure how to do that.Should we tell him well in advance to prepare him for it? Should we try to explain it when it happens, to try to minimize the time he spends worrying about it? Should we wait for him to figure it out and answer questions as they come up? I’m lost here. I’m sure – I’m hoping – there are options we haven’t considered.Disclaimer – My first “best friend” moved away after kindergarten, and it triggered the first of many depressive spells in my life. I was a far more isolated kid, and I don’t want to project my experience onto my son, but I want to help him manage the negative feelings more effectively than I did. Any advice is sincerely welcome!TL;DR - How do we prepare our four-year-old son for the imminent loss of his closest daycare playmate? What should we expect him to ask? via /r/Parenting http://bit.ly/2WvV5w3