Firstly, you are NOT a terrible mum! We all have days like that. Also, you're only starting out on your journey to find the tools and understand the uniqueness of your son's condition, and at first there is a lot of hit and miss. What works one day might not work another, and unfortunately it takes time and experience to build your arsenal of tricks. It sounds like it is definitely over-stimulation from being at the shops. Outside didn't work this time, it's a better tool for when they are reacting to a lack of stimulus. Perhaps if it happens again, can you try to get your DH to do deep pressure (squishing him between two pillows) making a game out of it. Deep pressure is meant to be calming it might work better at times like that. Even a bear hug could help a lot. What you DH says about a fit is actually how I would describe it too. My DD used to fit for real and she has a very distinctive cry that she only makes after having one, and also after she has calmed down on the occasions when she has gone so off the handle that I've had to restrain her during some meltdowns.

You can go see a private OT, no you don't need a referral but it helps. Also, does you community allied health organisation run a drop in clinic?

There is another tool to help identify sensory triggers so you know whether to add or remove stimulation, I'll see if I can find it.