I have a few opinions so take them with a grain of salt:

1. To paragraph 1: disengage. This kid is doing what half of american boys are doing right now-playing fortnite. It is the new normal and it goes hand-in-hand with him being a bratty kid because at that age (preteen) that is exactly how they are. You are expecting him to have the respect of a younger or older individual. That is not in his age range. They are indeed little shits for their preteen and teenage years. You should count your blessing that your slob wants to hole up and play xbox instead of running the streets impregnanting young girls or setting fire to buildings. 

2. You need to accept that just becuase you see his kid as a burden to you and society, HE doesn't. He is dad. He has rose-colored glasses on and more accepting to how annoying his kid is because that kid is part of him. So the whole "do you think he deserves a 1k computer" isn't going to compute with him logically. Of course he thinks his little snowflake deserves the best of the best and that's probably why he is as intolerable to you as he is now. DAD is your problem. 

I don't think you should just accept the teen for who he is or your DH for the disney parent he is seeming to be, I just don't think you should expect miracles out of the kid when the dad isn't laying the ground work. Shitty parenting = Shitty children. Dad needs to get that kid out of his room and outdoors, dad needs to make him clean up behind himself like a basic human being, Dad needs to see a financial counselor since he is prioritizing his kid getting expensive ass gifts just months before christmas rolls around even though rent is due. My advice to you is to accept they need work, you need to split finances and disengage from all skid responsibilities for your own sanity.

I don’t know what your financial situation is but in my world  , SO spending ONE THOUSAND dollars on ANYTHING without consulting me first would probably end the relationship. every dollar FH spends on MY FSD7 is a dollar not invested into our home, future and bio kids. I allow FSD to use my 1k computer when I am not using it , because she asks nicely and is very respectful of my home and property. my SO sees to this and also makes sure she thanks me and appreciates me. Not only is your husband doing your stepson a disservice by Disney parenting him into a spoiled brat and useless human, the spending behind your back seems to me a huge betrayal. 

It seems you and your husband have poor communication skills. 

My husband and I keep separate finances, but we still discuss large purchases, for ourselves and for our children (his, mine and ours). That said, neither of us vetos purchases because one of the kids doesn't "deserve" the item. That phrase isn't something I'd fathom. And if I heard it from my husband, it would likely be the end of our marriage.

Most teenagers are disrespectful, to some degree. Most teenagers are self-centered. A parent's job is to mold that child into a respectful, giving adult. But, it's not an overnight endeavor. 

Our kids all had laptops. In our area, it really is necessary for school. Is it possible that's where your husband is coming from?

Now, if that truly was rent money, and you're living paycheck-to-paycheck, and now you have no way to pay rent, then you have a bigger problem. Husband problem, not stepchild problem.

Well, this is all over the place. 

You need the money for rent, but in the next breathe running out to purchase multiple ipads, and in the next yakking about how you could have used the money on a boob job instead. Geez, which is it? 

Secondly, buying the kid a computer isn't necessarily automatically out of line. A ki in middle school and/or high school here wouldn't be able to do their class participation nor assignments without one. When people are putting out $800 plus or the newest version phone, an investment of $1,000 for a computer doesn't sound so crazy. 

As you mention nothing about household finances and whether or not you have joint accounts. or whether mutual agreement discussions are the norm of any and all money spend in your home or not... it's hard to say whether it's as extreme as the declaration of " how could he do this to me"  

I am 'allowed' to make some financial purchase decisions on my own without consult and/or 'permission granted', but then so is my husband. We both contribute to the financial needs of the household and each have both joint and personal cash. And no, we don't live paycheck to paycheck nor would a shopping spree of $1,000 but that big of thing. 

So, how long have you been with your husband that these things have not been pre-discussed and long ago agreed to? I would think household budgets and financial spending joint decisions, if needed, would have taken place prior to marriage. Is this out pour surprise the first time? 

However, with that said, yes, if the cash was really needed for rent and if in anyway affects the household lay out of cash , then , yes, he definitely should have consulted and if possible, went a bit more inexpensive.

Curious as to why you re the one doing all the 'parenting' of SS and fighting with skid over laundry and chores et? Last question, are all the children in the home his biological children? 

I would be furious. I think I would split finances. It’s one thing to “be sorry” and another thing to do something about it. Return the computer or move out - that would be my ultimatum. 

I was in your position a few years ago. I feel that pain. That pit in the stomach stress on why can I not have peace.