Player cannot join server anymore. Constant connection problem.

So recently, one of my friends is constantly having connection problems to my server. Me and another friend don't experience this so we think that it might be on his end. He used to be able to play just fine before though.
Here's the log: https://mclo.gs/KaTOzjD
Any ideas?

Most of the time, it's this error. Sometimes he just gets timed out or disconnected though. We also noticed that whenever he tries to connect, me and the other players get disconnected or have our connection timed out.