H20, as others here have wisely said, understanding what causes us to behave that way will make a huge difference.
I'm on the autism spectrum. I started out fairly unresponsive to people, limited repetitive speech, lots of repetitive movements e.g. rocking, total obsessions, couldn't go to social events etc etc. I'm now a mum and a co-owner of a lovely business. Life isn't easy for me, but it's worthwhile.
It may look 'orrible right now for you as a mum, but things can and do change for most of us. Especially now there's more understanding and help and support out there (never enough as yet, but there's some...).
You can probably imagine what it's like to be blind, or deaf? Imagine the exact opposite of that...a world where you can see so much detail and hear so much detail and feel and smell and taste so much detail that it's like living in an avalanche of the stuff with no way to switch it off - a bit like having every TV, radio, computer and speak turned onto full volume, all day, every day.
A shop might be interesting for most children. For many of us, it's sensory hell that hurts our brains so much as it overheats the brain wiring until we melt-down or shut down. Same principle if we don't know what's going to happen when.
And we can't see body language, hear tone of voice or see facial expressions/eye contact properly, so have no clue what you're trying to say to us or get us to do. It's just lots of movement and noise.
There's ways round nearly everything once you know what's what. Will it be easy? Nope, that's for sure. You need the right help and support, for you, for him, for his brother so you get some quality time with him too.