Stewart has been staying with Agnelli since fleeing from the home she shared with Pattinson, after admitting cheating on him with director Rupert Sanders, reported Radar Online.
"Rob knows Agnelli and has been calling him a lot since Kristen has been there. He wants to know why Kristen is staying there at his house? He is super angry now and has accused Giovanni of sleeping with Kristen too," a source said.
Angelli produced 2010's "Welcome to the Rileys" which starred Stewart and James Gandolfini.
"Giovanni got a call from Pattinson around four this morning, threatening him if he has been sleeping with Kristen. Rob is beside himself over Kristen's betrayal. No one has ever seen him like this," the source added.