Peter Andre is aging backwards as hits the gym with son Junior and they show off matching six packs

PETER Andre showed off his washboard abs as he posed for a topless snap with his eldest son Junior.

And the former pop star looked just like his offspring, who is almost 30 years younger than him.

The teen also praised his famous dad's youthful appearance and wrote over the top of their pic on Instagram: "I'm tryna look like you".

Junior's sweet tribute came on Peter's birthday, as the singer shared glimpses of his day spending quality time with his family.

He also penned: "Next is my workout with my son. He did give me a hug but it was more of a shoulder tap!"

Fans couldn't believe Pete' age as he celebrated being in his fifties.

The Mysterious Girl Singer was showered with love from his family and friends on social media.

Peter's daughter Princess posted a story for her dad on Instagram.

The 16-year-old shared a sweet snap and captioned the picture: "Happy Birthday Dad, I love you so much."

Fans were quick to send their birthday wishes to Peter with one fan commenting: "Happy Birthday for today @peterandre my birthday twin!"

Others were shocked by his age with one fan commenting: "I can't believe you're 51 already you still look so young!"

About his birthday plans, Peter wrote in his column: "Emily and I celebrated at the weekend but my actual birthday on Tuesday will be a low-key event.

"The kids, Emily and I are all going out for dinner to mark the occasion and I've left it up to them to decide where we go.

"I don't mind as long as we're all together and then we'll come home and take it easy.

"Things are a little different to last year - for my 50th we went to Dubai - but obviously we've got the baby coming in a few weeks. That's the greatest present I could ever ask for!"

Peter who currently has four kids is about to be a father to a newborn again.

He shares two kids with his wife Emily, Amelia, nine, and Theo, six.

He shares Junior, 18, and Princess, 16, with his ex-wife, Katie Price.

Emily and Peter announced they were expecting a baby last October.

As Emily's baby bump grows, Peter has been attempting to empathise with her. Most recently, he strapped melons to his body to experience the sensation of pregnancy and better understand his wife's perspective.

Fans found the comical Instagram skit hilarious.

One fan commented: "Haha I love it….cheered my morning up. Peter well done for giving it a go."

Another added: "Never laughed so much for ages- what a good sport you are Peter."