If it were me I’d make sure he has a way to stay in contact with friends to make plans for the park and such. Other than that… there isn’t much to it. Maybe not give it too much attention either kinda thing. It may be a smoother transition if you acknowledge the change but more so it’s simply what is happening. As for a gift for the teacher I’m a fan of the child writing a note that is personal and then, maybe, a small parting gift but absolutely not necessary. I mean really small like a few dollar nice pen or a chocolate bar. If they don’t want to write something just making sure they say a proper goodbye is always good enough too.