An atrocity has occurred in your home. Your shock is understandable, but you cannot afford to passively turn away and bury it. Your irresponsible, lying, manipulative Partner abused DS by exposing him to pornography, and he is trying to cover it up.
I think DS changed his story because your repeated questioning suggested to him that his truthful answers did not suit you, so he was embarrassed and deflected elsewhere. It is also possible, as others have surmised, that P has been whispering in his ear.
Although it was crucial that you examined the iPad asap, you left it where P was able to take it and delete the evidence. Please secure the device and take it to a computer shop yourself. If P removes it from the house, he will sabotage any further investigation.
P portrays himself as a vocal homophobe. Yet when his young son viewed male sex, “he wasn’t overly concerned and said he’s bound to see stuff with technology these days but he would put a filter on it.” P downplayed this serious violation of DS because he is responsible and wants you to back off. That is why he is now accusing you of causing drama by searching the videos. He is manipulating you.
What were the videos of, OP?
You want DS “to forget about it,” but that is dangerous thinking. He won’t forget what he saw and does not have the emotional tools to deal with it. There is a reason this is called abuse. Please seek professional support for him.
Your P’s actions harmed your little boy. You must decide if this is a dealbreaker for you. It would be for me. 