Hi Etchasketcha
I have some thoughts to share but I’m not a step dad so hope they help. It would be good to know some more info like how long have you been stepdad? Has the 12 year old always been like this with you even when you were dating mum? You say he shows you no respect but is it just when you give him firm orders? He is 12 and is growing into a man himself - part of his teenage years will be to push at authority figures its natural. You might like to ask dads of other 12 year olds what their experience is like you might find the step dad perspective is not the real issue it just feels that way to you.
I agree with Casanova that your ages are close and he might be doing the young stag thing of bucking up against slightly older "peers" (12 years isn't much I have friends who are 12 years younger than me! My wife is 9 years younger than me)
When you say he jumps on you what do you mean? Is he aggressive? The throwing thing is to be expected is some sense, but of course having said that it depends on what he's throwing and where he's throwing it! Studies show that 2 year olds and teenagers have a lot in common psychologically, for a 2 year old temper tantrums are normal day to day occurrences, expect a few during his teenage mood swings.
Try and look at each situation that he challenges you on separately, ask yourself why does he challenge that i.e. the bed thing - what time of night are you asking him? If it’s before 8.30 - 9pm? If it is then he will kick off as at 12 he should be allowed up until then! or at least be in his room reading etc. In each situation he is pushing his boundaries, he's 12 he is meant to be doing just that. Your role now is to help him. In other words pick you battles dude. Give him some space, allow him space to make mistakes as long as you and mum help him pick up the pieces and teach him as you do.
In giving 1 to 1 time with him you are doing a great thing - something thousands of other dads are not doing. Give yourself a medal for giving him that time it will reap rewards and builds trust and love. In your 1to1 times try revisiting situations and asking him questions about what's going on, i.e. the bedtime stuff, why is he kicking off about that, the reasons he gives could be simple to rectify.
I'm writing a flippin essay here sorry 😆 ..... In essence pick your battles mate and try talking to him when he's calm and happy!