hey Chic, hang on there, overthinking might be contributing to your anxiety. It's YOUR house, you do what is needed. He is simply throwing a tantrum and not getting his way.
I'm sorry that you are finding a tablet difficult, it offers you portable options, buying one should not be a big deal, I went to Best Buy and picked the young salesman's brain, he spent time showing me how to do things, then I ordered it from amazon, cheaper and at the time, no tax....accessories much cheaper too.
it's not your son's business how you spend your hard earned money, he certainly is not contributing to the household.
you have nothing to feel guilty about, he has had a very soft life and keeping it that way won't help him or you...You have reached your saturation point, the sponge is soggy and dripping. You said enough and stayed strong, he is holding his breath waiting for you to stop him from turning blue.
He is very smart and manipulative, he is hitting below the belt with the nasty remarks, you expect that from a 12 year old not an adult. He wants what he wants, he does not see the big picture. You do.
I am sorry he is continuing to be nasty, but you have a thick layer and fur to protect you (kind of like fat Fandy, but you are not fat)...let it roll off your back and keep moving forward.