What would you do?

My sons father and I have shared parenting. My son is with me two days then his father two days then with me five days then his father five days and repeat. Shared parenting started in January. My 3 yr old son comes home saying very inappropriate things as well as doing but his father refuses to talk with me about or insists I’m crazy. (Humping the air, saying “I’m gonna funk on you” ,touching himself laughing like he’s doing something he shouldn’t and when asked about these things he completely changes the subject or says he doesn’t know. Last night he said “I’m gonna fuck your butt” he came home from his dads last night. He starts school in August and can’t be speaking or acting this way. What would you do?