No, its not normal, and you are right to be concerned.
Unfortunately it sounds as if your DP is being willfully blind, and what he does see, he's unwilling to do anything about. You cannot win this one. Disengage as much as possible. Anything that your SS wants/needs done, let DH do it/sort it/pay for it. You focus on your toddler and your relationship with DH. As much as possible, step back from your SS and get some emotional distance.
My SS is nearly 18. I've raised him as my own since he was 7. He's always had a bit of a cruel streak (which he's careful to hide from DH), and the ability to lie convincingly through his teeth, but up until about 8 months ago, things were good. The odd hiccup but nothing out of the ordinary for any kid.
Then he got a girlfriend and since then, his behaviour and attitude as gone down hill in a big way. He treats his family with utter contempt, (with the exception of DH, he hasn't crossed that line just yet), to the point where his sister and step sister no longer speak to him. He went to his GF's place in the afternoon of Xmas eve (telling DH he'd be back in the morning) and that was the last anyone heard from him, or saw him, until the evening of Boxing Day. He didn't even phone DH on Xmas Day. When he did appear he threw a massive tantrum when I remarked that it was nice of him to finally grace us with his presence. It's fair to say we had a free and frank discussion, one of many in the last few months I might add.
It's made very little difference. DH has always viewed his boy through rose tinted glasses. He's always been softer on him than the others, makes excuses for him, says he's going to talk to him "man to man"... never does. The kid gets a slap on the wrist with a wet noodle and that's it.
Last week I was the target for his contempt for the first time and to be honest, I was shocked and deeply hurt. Like you, I've bent over backwards to help him, to be there for him no matter what. To guide and support him financially as well as emotionally. I've tried to not mind DH's inability to see how awful he's become and his unwillingness to do anything about it. He talks about it, but it's just that. Talk. As far as DH is concerned, boys will be boys, it's harmless, he didn't mean it, he'll have a talk to him, he was just as bad when he was a teenager etc etc. Excuses and no real consequences. Every. Single.Time.
However, I'm not having a bar of it and have stopped paying for things that I have provided for him in the past (like his phone plan, like Spotify etc). DH initially agreed, then backtracked. Too late. It's done. SS is now on a pre-paid plan, which SS will have to fund and Spotify is gone. If he want's it back, he can pay for it himself. As I said to DH, SS has known me long enough know that I do not tolerate rudeness or disrespect and he surely can't be stupid enough to think that he can treat me the way he did, not apologise, and there be no consequences. It ain't going to happen. I haven't told SS, I'll let him work it out on his own.
It's an awful place to be in, and as I said, you can't win. If you say anything to your DH he'll instantly defend SS and resent you. If you don't say anything, nothing changes. Disengage, and good luck.