The 47-year-old actor and the 'Jennifer's Body' star announced in May that they had broken up after almost 10 years of marriage, and it didn't take long for Megan to strike up a new romance with musician Machine Gun Kelly.
And although Brian has enjoyed a series of dates with models Courtney Stodden, 25, and Tina Louise, 39, since his split from Megan, he's said to be irritated by the publicity of Megan's new relationship.
A source told People magazine: ''Brian is annoyed that she is so focused on her relationship with Machine Gun Kelly. He doesn't understand the need to post social media pictures with silly captions to proclaim her love for him. They aren't even divorced yet.
''It definitely annoys Brian. And it stings a bit too that Megan moved on so quickly. Megan was always very private, so Brian just doesn't get what changed.''
The news comes after Brian - who has Noah, seven, Bodhi, six, and Journey, three, with 34-year-old Megan - recently defended his decision to date multiple people after his marriage split, insisting he's not a ''s***** person''.
He said: ''What people normally do when they get out of [a relationship] is they date. They talk to multiple people at once, some people use dating apps, they talk to multiple people, they go on multiple dates, they spend time with people until you feel a connection with someone then something builds from there.
''I'm not playing anybody. My life is just much more under a microscope than somebody else's. Isn't that dating? Aren't you supposed to talk to multiple people? It doesn't mean I'm a s***** person. That's what you're supposed to do, meet people and experience life. When you start finding commonalities with someone, then it becomes something bigger. You have to be willing to get out there and put yourself out there. But I get judged for it, I think, unfairly.''
And Brian insisted he only wants the best for Meghan in the future.
He said: ''Everyone still loves and respects everyone, things are just different. It's not bad different, it's just different. I wish Megan the absolute best in everything. I want her to be completely happy, for her, for the kids. That's super important. No one wants to be around somebody that's unhappy, nothing good comes out of that. That's a terrible situation. We are taking it day by day.''
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