Chinchilla suddenly became aggressive

Hi all, I've had my male chinchilla two years now. A few months ago (4 or 5) I adopted another male who is about ten. My younger guy is being incredibly aggressive towards me and other people. He's completely fine with the other male (they regularly have playtime but don't yet share cages). The younger chin lunges, bites and chatters his teeth whenever I go near him. The only thing I can think of is that he is angry at me for giving him some eye drops the vet prescribed. Any help with this would be amazing, I just want my Oaks to keep attacking me :(