My gfs son is acting very strange I need answers please ! ?

I live with my gf she has two kids here son Hayden is 4 years old he is a pale boy with red hair . For the 3rd time now I will come downstairs and catch him with his pointer finger in his anus then he will pull it out and suck that finger I screamed at him not to do it bc I find that extremely ****** up! And wen I look back he will be doing it again and if u yell at him he just gives u this creepy *** smile and laughs . He also enjoys hurting and tormenting his 2 yr old brother and puppies . I have a 2 month old husky and out of nowhere he enjoys kicking it pulling on it and punching it for no reason and smiles and thinks it's funny . If he doesn't get his way he will freK out unlike anything I've ever seen he will shreik like a girl and throw his arms around vigorously while jumping in circles then if u lock him in his room for timeout he will trash it!! If you yell at him at times he will talk very creepy and spit all over his face . The worst thing is it's like he's schizophrenic he will be the best boy in the world then the creepiest . His mother thinks it's add or ADHD I think not bc I had both and so did many of my freinds and never once did we act anything like this . Please help so I can know if this is something I should get help for right away ! Thanks