I might be unpopular here but I say YTA. It sounds like your stepson is trying to connect with you in his own way. You need to understand that as the step father, the onus is 100% on you to reach out and build bridges when there is opportunity. I don’t see anything wrong with an old sock in a box, that seems like a really harmless prank and sounds like it was meant to provide a good laugh between the two of you. I don’t have enough context for the other pranks but it’s valid if you have concerns of your safety and it’s reasonable to draw the line on egregious ones. You need to understand that having to adjust to a change in family dynamic such as divorce and resettlement with another parental figure has a lot of impact on a child and can take many many years to process, especially when it occurs during these crucial formative years of being a teenager.