As well known, boxers are goofy hyperactive dogs that need a lot of excercise. If they don't get their exercise, they will behave oddly. So start with giving them ample exercise.
Also keep in mind that keepers are not always the best trained to take care of dogs. Sometimes they might be doing things to aggrevate the dog, intentionally or unintentionally. So try talking to your keeper to understand his attitude towards your dogs. If your keeper is sincere, you can ask him to create a log of when the dogs attack him and see if there is something disturbing your dog. Once you have identified what bothers your dog, you could either elimiate the stimulus or get your dog used to the stimulus gradually.
If your keeper does not come across as sincere, change your keeper immediately. Bad keepers can completely destroy your dogs behaviour and make it extremely unbearable to everyone. I had a dog keeper who would keep irritating my dog and get him to lunge at strangers. It got so bad, that my dog started biting strangers on the road all the time. So please be careful with keepers. if required, get your keeper to attending dog training, along with your dog.
Also, most research points towards spaying /neutering your dog calms your dog down and is also very beneficial for your dogs health and avoids several cancers and complications.
And last, but not the least, socialize your dog. If your dog is comfortable with you, gradually introduce your dogs to the keeper. Start with keeping your dogs on a leash and constantly praising and treating him, while your keeper approaches your dog a little at a time. Increase interactions between keeper and dog gradually. It might take some time at the begining, but it pays off at the end.