How Do I Handle 11-Month Old Chihuahua Aggression?

I have been very consistent in training my chihuahua, who is now currently 11 months old and 9 lbs. He has been with me since 12-weeks and was neutered as a young puppy as he had started to exhibit certain behaviours (humping objects, etc) that led it to be advised. He is crate trained, potty trained, social and friendly in the dog park, but has exhibited aggressive behaviours from time to time. He is not allowed on the furniture, is made to wait for humans to enter doors first, and must sit or do other tricks to be rewarded or served his food. However, his aggression has been most seen around food, guarding his crate, guarding me, or when people pet him. Before, he would always want attention from others, but now he will be comfortable receiving a pet but then suddenly turns and snaps, warding off whoever is petting him. My boyfriend, who lives with us, is the most frequent victim. This past weekend, after getting picked up from the train station, I was sitting with my mother and the dog in the back seat of the car. She was petting and he suddenly tensed up and tried to bite her. Prior, he had always been very calm and well behaved in the car. On another occasion, while sitting on my boyfriend's lap in the car, my boyfriend was holding him and moved his hands slightly and the dog began to growl. I'd don't know if this is fear based or territorial. But the behaviour is very fickle and changes quickly. He will be accepting one minute, then suddenly tense and upset. He has been examined recently with a good bill of health. This behaviour is never exhibited towards me. After he lashes out, he instantly gets embarrassed, will lower his head in submission, and know that he has done something wrong. But he will still possibly lash out if anyone is in his space. I'm worried as this behaviour is new, as in the past few weeks, and I want to nip it in the bud. In regards to his aggression in the crate, I've had my boyfriend casually drop treats into the crate when the dog is resting inside, and speaking kindly to him. This is slowly improving and he is more accepting of people's presence near his crate. However, the other moments when he snaps are concerning me. If anyone has any advice-- please let me know. I have also been seeing a behaviorist to assist, but wanted to gather additional feedback.