8 month old puppy suddenly aggressive after neuter?

My chihuahua mix puppy was neutered about two weeks ago. Before his neuter, he was such a sweet boy; loved affection, played with his cat brother constantly, and was generally good meeting other people and dogs. He had a little bit of an issue with food aggression but was trending in the right direction after some training. After his neuter, he’s suddenly been very aggressive. Snaps at our cat constantly, especially when he gets near his food. He’ll be fine on certain days, acting sweet towards me and my girlfriend, then randomly switches up on us lunging and growling, and even bit my girlfriend which he’s never done before. We’re starting to get very stressed and miss our sweet boy, has anyone experienced something similar? What kind of training can be done for aggression when it’s seemingly random? Is he going crazy with boredom after resting and needs more mental stimulation? Please help!!