There can be a post-neuter craze due to hormonal adjustments following a neuter which can exacerbate hormonal behavior (e.g., more aggression). That being said, it’s important that you also consider whether this change in behavior is an expression of stress due to a health or frustration/boredom issue. Since he is eating, drinking, and pooping fine, I’m inclined to think this is within the post-neuter craze, especially if his neuter was within the past month.
Fast breathing can definitely be normal based on context. If at any point it appears he is having trouble breathing, then there is definitely an issue. You can also consider, though unlikely based on time of year, if rapid breathing is associated with being too hot.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.