New York, May 31 -- 'Back to the Future' actor Michael J. Fox made sure to send birthday wishes to his eldest child, Sam Michael Fox, on his special day.

Taking to Instagram, the actor shared a light-hearted photo with his son, where the two are seen standing close, and Fox is playfully pretending to punch Sam in the face.

Along with the picture, the actor added a caption full of love. It read, "Happy Birthday, Sam... No better friend, brother, or son on the planet. Love you so much," wrote the 63-year-old actor.

Sam is Fox's only son. He also shares three daughters -- twins Aquinnah and Schuyler, 30, and Esme, 23 -- with his wife of over 30 years, actress Tracy Pollan.

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