Where is his mom in all this?

I don’t recommend you diagnose this child. But I recommend mom signs him up for therapy. He has no father, mom and you got married but your marriage is difficult. I know you might say kids don’t know what’s going on between you two. But they always do. In addition the way you described your wife, perhaps her son is taking after her, being difficult.

So there is a lot at play. Mom needs to start some parenting and signing him up for therapy and you might need to back off a bit. You’ve been only married a year. This kid might not see you as authority so being oppositional possibly is his natural response to you giving directions