Sebastian Stan Is ‘Smitten’ With New Girlfriend Jennifer Morrison (EXCLUSIVE)

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Sebastian Stan has always had a thing for his leading ladies, only this time it’s serious.

The Political Animals hunk has been quietly dating his Once Upon a Time co-star Jennifer Morrison and friends haven’t seen him so taken with any of his girlfriends quite like this – even when it comes to his other famous ex.

“Sebastian is so happy,” a source close to the pair told Celebuzz.

Who is the other famous ex-flame?

“No one has ever seen him so smitten — not even with Leighton (Meester).”

Stan’s first high profile relationship was with Leighton Meester, who he met on the set of Gossip Girl. The pair dated for two years before splitting in 2010.

As was reported in July, Morrison, 33, and Stan, 28, have been seeing each other for over four months – and Celebuzz can reveal the happy couple is spending more and more time together.

And it looks like they may even be getting their own fairy tale ending.

“They’ve been in New York City together all summer,” said the source.

Like Stan, Morrison has also gotten close to another former co-star. She was engaged to her former House M.D. castmate Jesse Spencer.

Captain America star Stan most recently dated Glee’sDianna Agron.