Why is my friend being so immature?

Now I know that my friend has ADHD. But the thing he does are really annoying. Like he sometimes speaks in a high pitch voice to sound like Alvin from "Alvin and the Chipmunks." The thing is he's telling me something about a guy named "wondismo" but I can tell he made that up but he just doesn't care. Everything I tell him to stop he just keeps on forgeting. I have no idea what to say to him now. Plus the other day he called me and told me things about what is gf is doing and its really none of his business or my business. I feel like my friend is not acting normal. What can I do? Should I stop being friends with him? or Should I find new friends that are normal and nice?