I would place him with a sitter who has no other dogs, must admit I would be most upset to find my dog being met by aggression, surprised the sitter is still taking him. Most wouldn't want that, it's not their job to have to sort things like that out and the other dog owners would be livid if they found out.
In fact if he even attacks your own dog (or is it rough play fighting, different breeds play in different ways?) but regardless if she is old it's no good I would house your AC in a foster home alone and leave your older dog with this sitter for company if she is happy.
Dealing with the aggression/heavy playing is something for you to work on at home, not a job for a sitter to do IMO.
Sounds like he needs a firm voice and distraction techniques, make sure he knows you are the authority figure here, strong eye contact, firm calm voice and trained commands are needed.
If you can find him a sitter who will also back up your training on a one to one basis it will help a lot whilst you're overseas, I wouldn't rely on other dogs putting him in his place it could cause more aggression. It's acceptable in a pack but other dogs are not his 'pack' he will just view 'other' dogs as aggressive, turning him one of two ways.
For now, until he settles with age and training (hopefully, time will tell) IMO he should be with a sitter with no other dogs. (I'm thinking of the other dogs, I wouldn't want my dog attacked)