One Friend Can Join my Server, but Another Can't

My friend is having trouble staying connected to the server and also just connecting to the server, Sometimes he can connect then it'll kick him and he can still walk around for a bit before getting disconnected and it says "Internal Exception: java.io.IOException: An existing connection was forcibly closed by the remote host". Sometimes he can't connect at all and it says he Timed Out. My other friend can still join with no problems at all. He also tried the DynIP and it still won't work. He can still join other servers like hypixel.