Sounds like ADHD to me. My son talked about the weather a lot too. Maybe instead of getting annoyed try letting him learn as much about the weather as possible. Get him some books on tornadoes, storm cells, meteroerology, etc. it just may be something that really interests him. My son talks about cars too, all the time. What kind of car is this, that etc. when he is on his meds it is not as bad, but here's my deal, if it is a strenghth with him unlike other things like math, i let him go with it and fill up his brain. Maybe he can be a weatherman, who knows. I know the talking is annoying, but thay really can't control it. Go online to additude.com read the magazine look up ideas. Good luck.
What do his teachers do in school? I'm pretty sure they don't let him talk nonstop all day long.
Maybe you could get him to start texting you? Or making videos which he could send you? Make sure he adds lots of weather graphics and charts to the video.
But seriously, I really do wonder how he is doing at school? Does he have friends? Can he relate to other kids, etc?