No need to be embarrassed that you cannot afford an expensive trainer. I’ve has a lot of ups and downs this year, and a lot of that affected me financially. There is no shame in admitting that you know your dog needs help, but you are not in a financial position to do so.
I think the biggest thing here is the cause/direction of the aggression, and the unknown for us reading. Is he aggressive towards you and your son? You mentioned him being too sensitive to groom. How does he react when you go to brush him?
Lyka did the same “I’m cool” thing when people were sitting, and then chased them out with ankle nips when they got up to leave. It’s fear causing that. They are okay when the person is sitting because they don’t see anything threatening. When they stand, however, it’s putting him into a panicked flight or fight mode, and he’s going for the fear fight. Get them out of my house as quickly as I possibly can. This is easy to work out, but time consuming.
Biting the trainer? I’d have wanted to bite him too. Let someone three times your size flip you and hold you in submission, and see if your natural fight instincts don’t come out. Biting the tech that muzzled him, not great, but not the worse thing that has ever happened in a vets office, I can promise you that. But what was the outcome of the bite? Did he puncture skin? Or was it a quick warning mouthing going on? If he showed bite inhibition and didn’t apply pressure behind his bite, it’s not a dog I would even consider euthanizing.
What have you done training wise yourself? Do you do continued training with him on a daily basis? Even my 9yr old still gets a kick out of her short 10-15 minute daily sessions.
Not sure what info you have been messaged, but keep in mind, even members on this forum with a ton of experience contradict each other, and some seem like they know their stuff, but have questionable methods. Just be very choosey in who/what advice you’re taking, including my own! We all have different experiences, different directions we want to take our dogs in, and some know a lot about the breed, but have only worked with puppies from reputable breeders, so they started off on good footing. Some have only had rescues and wouldn’t know what to do with a great breeder pup because they are so much easier than working with nerved dogs, that training can often fall to the weigh-side and they let them “train themselves.” And of course, everyone in the middle of the two as well.
I really do wish you and your guy all the luck, and hope you can find a way to get him in a better place mentally, or physically if it’s something you just can’t take on yourself.