I think it's important to understand your brothers weaknesses, in order to help him meet his full potential. Your parents are still in that steep learning curve, as is your brother, in that they don't know yet what his triggers are. Anxiety has a huge role to play in Autism, as does Sensory Overload. Once your brother/ parents can identify the weaknesses they can establish interventions/ coping strategies that will help.
I have a nine year old Autistic son, he was five when his Autism was first identified. It took time for us to adjust and implement coping strategies. We went on several courses on anger management, sensory overload and parenting a child on the Autistic Spectrum. I also read a vast amount of literature on Autism. What we have come to understand, is that in Autism, there is always a reason for behaviours. Sensory overload is a very frequent trigger in Autistic individuals. It is honestly torture just leaving the house for many people on the Spectrum. These links take you to different autism simulations, that will hopefully help you to understand why your brother would rather stay in his room than go outside.
I honestly would want to stay locked in my room too, if I had to experience the world the way my son does!. My son didn't ever want to leave the house, he couldn't cope at all. It wasn't until we realised the impact that sensory overload was having, that we were able to look at different aids to help him. My son will go out now, in fact he loves going out but he needs to have Sun glasses on at all times. He takes his mp3 player and head phones that cut off the noise from outside. He will wear a baseball cap with a wide rim on sunnier days. My son can still get overwhelmed by smells and too many people, even with his ear defenders/ headphones and sunglasses. Which is why he will spin/ flap his hands/ vocally stimulate making loud noises to drown out the noise that headphones haven't drowned out. This constant attack on the senses is absolutely exhausting. As soon as he gets home he is straight under his weighted blanket with his vibrating pillow and the lights all off. If he can't have this time to recharge he will meltdown/ shutdown.
As for the mess, there is actually likely a valid reason for this. Because the world is so overwhelming, autistic people need to be able to control their environment. My son doesn't like change or surprises, if we want his room tidy we need to make sure we have put in his laundry basket and bin where he can see them! We have built putting his dirty clothes into his laundry basket, into our sons daily routine. There are different apps online that people on the spectrum can use to help organise their day . My son needs to know in advance what is happening, we often make a schedule of events, so say there is a family event, we would tell him how we will get there (car), show him where we will be going, tell him who will be there, tell him what we will be doing whilst we are there, then tell him what we expect of him whilst we are there and most importantly when we will be coming home. We also take a bag with his mp3 player, his ear defenders/ headphones and his ipad with sensory apps. As your brother is older your parents can help him create his own schedule and itinerary.
There is a lot of change going on in the teenage years and it becomes very overwhelming for autistic teenagers who are already struggling with sensory overload, social skills and friendships/ relationships. They can just want to retreat from the world and escape to a land of books or computer games. I think it is wrong to lower expectations and allow them to become complete hermits. But I can understand the attraction this has to both the Autistic teen and your parents. There are strategies and coping mechanisms that will help them to push themselves and integrate more with the neurotypical world. But it will take time, research and communication to help find the strategies that will help your brother. There is a fabulous quote in the Autistic world ' if you have met one person with autism, you have met one person with autism' !!!
I think you are being unreasonable OP, but given that the diagnosis is recent it is entirely understandable. As for the eating, that is definitely a teenage boy thing!
I would try to take a look at what your brother is spending money on, is it apps, games, etc? Or is he trying to impress friends? Teens on the spectrum can be vulnerable to exploitation so it is worth looking at the cause of the spending.