My friend tries to connect but disconnects after a few seconds

Hi, I host 1 world that consists of 5 players. just 2 days ago my friend got disconnected after we took our first ride using the elytra in the overworld. We completed the end a couple mins before that, hopped back in the overworld and his connection was still good. After he got disconnected, he tried multiple times to reconnect, but failed and its been the same result until now. I would see him appear in the world floating one block high, drop down after a few seconds, walking around and trying to open doors for a few seconds until the door doesnt open anymore then he gets disconnected. He would get the "java exception" error. he's able to connect to other servers like hypixel with no problem, which means his internet must be fine. This is very strange since me and the 3 other players are able to connect to my server as usual, not having his problem. I use paper as my software, i have no plugins installed, there are no OPs, and he still cant connect. i checked the logs and his errors would appear as "kicked due to keep alive timeout" seen in the attached link. or from yesterdays log and the day before it would also say "cant keep up, is the server overloaded?". i tried downloading the world, uploading it again but its the same result. I could really use your help to solve his problem cause its a pretty developed world and we dont want to lose it due to one of us not being able to connect.

https://mclo.gs/DFQ7zUb

We went and tried the dynip, still cant connect.
he tried the changing his DNS to those google and cloud fare DNS but still can't connect.
Could there be another way to fix it?

Quote from AlexanderSniper1

Tell your friend to try to connect with the dynip https://aternos.org/server#connect

If that doesn't work tell him to change his DNS