We basically have an issue between DDog1(7) & DDog2(2). Both are entire male working cockers. We also have DDog3 who is a year old spayed staffiexwhippet.
DDog1 is very badly damaged due to a bad start in life. He's very timid which lately tends to manifest itself as fear aggression towards other dogs. If provided with a way out he will always back down ie- he's not looking for a fight.
The big problem is that he's started to get very jealous of Ddog2 and we have had several spats where he has attacked him. No blood has been drawn but his meaning is quite plain. On occasions he can be seen staring him out, tail stiff, body tense and a fairly aggressive stance. DDog2 does not respond aggressively but will try and defuse the situation by backing down. DDog1 has never been aggressive to DDog3, and she is a typical boisterous pup who is always "in your face" IYKWIM?
We only have a small house and segregation is not really possible (unless we crate one at a time) I suspect it's because they are both entire that the problem is occurring (springtime, DDog3 has recently had an issue with recurrent seasons which upset the harmony somewhat 
)
Would castrating DDog2 help?FWIW, neither are particularly masculine dogs.
If necessary I will consult a behaviourist but our couple locally are slightly dubious so would need to go further afield.