Am I tripping

So I’m in a relationship with a guy with a 4 year old son. I started noticing maybe a month in his son did little sneaky things like waiting until his dad left a room for a couple minutes and he would try to play in my face for example he would take toys and mush them in my face. I didn’t say anything to the dad because I’m thinking maybe this is just him trying to play until he kept doing it only when his dad left and stopped when he came back. It gradually got worse when I would have my nieces over around the same age as him & they are telling him to stop and he’s laughing in their faces and not stopping & I’m having to tell him to stop. I told his dad his is picking and being sneaky. He gets mad at me and tells me his son doesn’t have friends and that’s how he plays….i start to get irritated and mad because.. he barely disciplines his son at all.. He had to tell his son multiple times to sit down.. stop doing this.. stop doing that. I mind my business when it comes to him parenting his son.. but when it starts to have an effect on me and whoever I bring around it’s a problem. I’ve expressed to his dad on multiple occasions something has to be done.. his son lacks discipline and boundaries… he only does sneaky things to females.. I even caught him trying to hump my niece.. they haven’t been over to play with him since being that I don’t ever want to endanger my nieces etc.. I get he’s a kid & me and his father don’t have any other issues besides those things. I’m stuck between a rock and a hard place because I’m close to leaving the dad alone because of his son’s behavior.