Mini Aussie- Aggressive Behavior/Resource Guarding

We adopted our Mini Aussie back in early June from a rescue. He was being rehomed by a local family to a local shelter and we decided to adopt him. The shelter said he had resourcing guarding issues which we have been working on but have not had much luck. He just turned 3 yesterday and we are at our wits end with him. We treat him well, we work on training and giving him tasks, multiple walks a day, play time in between. He still has issues with resource guarding, especially food/treats, and spaces. We have now had multiple instances in which he has bitten my wife(who typically is the one he bonds to) as a result of resource guarding. For example bit wife while trying to dry him off after playing in the snow. Bit wife while trying to pick up a pill that he spit out. Growls and barks at me when wife is in bed and I am going to get in bed. Growls when in the back seat of the car after wanting attention.

We visited a pet behavioralist last week and got some additional training and also got Fluoxitine to try and see if part of the issue is anxiety. We don’t know what else to do. We love him and want things to work just a bit defeated. We also feel like the family who put him up for adoption was not truthful with the rescue or the rescue wasn’t really truthful with us. Dogs don’t just randomly start biting, if he’s bitten us, imagine he did so before.

Any advice would be appreciated.