Change in behavior after new addition to herd

Hey everyone! Looking for some advice for my 15 year old gelding. Has been kept for 1 year with another mare and gelding on my farm and seemed low man in herd. Since we added a new gelding last week, we have been experiencing some bad behavior (refusing to lead into barn before other horses, rearing in response, and this was rearing in stall when I had to give him his dose of ivermectin). We had to hold him in a corner of the stall with 2 people on either side to give meds but got it done. Can I stop this rearing when he decides he doesn’t want to cooperate? It’s incredibly dangerous.