My dog had a castration last month and now.....?

With his being a rescue you can not know his genetics and as much as Pitty lovers (yes that includes Staffies) hate to admit it. Genetics play a huge role in their adult temperament. He is reaching the age of change and often it is between 18 months and 2 1/2 years that aggressive traits appear. It is likely that your neuter was late for his hormone surges and the traits just happen to coincide with his neuter. You must get a professional trainer to work with your dog now before these issues become dangerous.