Brian Austin Green previously revealed he and wife Megan Fox separated late last year

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Brian Austin Green has been letting his discontent with estranged wife Megan Fox be more known.

The former 90210 star recently posted an Instagram slideshow of his sons with the same caption Fox used for a photo of new flame Machine Gun Kelly: "My heart is yours." As a source tells PEOPLE in this week's issue, his behavior has to do with how he feels about her moving on so quickly — and publicly.

"Brian is annoyed that she is so focused on her relationship with Machine Gun Kelly," the source says. "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," the source continues. "Megan was always very private, so Brian just doesn't get what changed."

  • For more about Green and Fox, pick up this week's issue of PEOPLE, on stands Friday.
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Brian Austin Green; Machine Gun Kelly, Megan Fox

The couple share sons Noah Shannon, 7, Bodhi Ransom, 6, and Journey River, 3, whom Green, 47, has been caring for as Fox, 34, wraps up filming her latest movie with Kelly, 30.

Green previously announced he and Fox had separated late last year after she came back from filming a movie overseas and asked for space. They married in 2010 after meeting in 2004 and getting engaged in 2006.

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(from left the right) Machine Gun Kelly, Megan Fox, Briand Austin Green

Fox was first linked to Kelly back in May when pictures of them spending time together started circulating.

In a recent podcast appearance, Green opened up about how he's handling the high-profile relationship.

"You can if you ignore it and avoid it which is what I try and do," he said of the pictures and reports of Fox and Kelly out and about. "I try to no read anything or look at anything and not involve myself in it. Just keep my head down and do what I do and focus on the kids. I know she’s going to do the same thing."

"Everybody is judgmental right now that she’s out of the country working and she’s not around," he said of Fox, who's finishing up filming her movie with the rapper. “ We travel and we work. It’s no different than before, we’re just no longer together. This is what we’ve been doing for 15 years.  When one person is traveling and working the other person will step in and take care of things at home and the kids and this situation is no different. It’s not for lack of caring. Megan loves her kids, would do anything for them, and has and will continue to."