My friend gets kicked out of anything I try to join him in

On ps4 me and my friend used to be able to play together but then ps4 got the ddos attack around the time last of us came out then we started having problems where if I joined a party he was in it would kick him from it the only exception of games we can play is eso and bf hardline we can alook play normally when I'm in my grandma's house which is a different internet provider how can we fix this?