nasty intrusive thoughts

Can anyone give me advise please. My son who is almost 11 always has something he overthinks about whether it's what he's touched or what the dot is on his food etc however, this overthinking things has taken a turn for the worse this week while he's been off school It started while we were playing football. He kept saying "If I don't save this goal my family will die" that's progressed quickly to playing fifa or watching a film and him saying his thoughts tell him that he wants those people to die. He's now saying his thoughts are towards - his dog, me his mum and his dad. As you would expect he is getting very upset by this. As fast as he's telling me that is his thoughts he's saying but I don't want anyone to die why am I thinking this. It's breaking my heart to see him going through this current phase, and I know its a phase because they all are whatever subject he's overthinking. Has anyone else had any experience of this and what do I do for the best. thank you in advance...