Katie, 42, shared a video of Harvey, 18, showing off his handmade gift to Junior.
His handmade card includes a recording of him playing Happy Birthday on his keyboard and singing along.
The card reads: "Junior, wish you a happy birthday. Love from Harvey xxx".
And he drew a massive number 15 with a picture of Junior on top.
"Junior is going to love that"
Katie asks him: "Show me what you've done?"
And he proudly holds up the card as it plays the tune.
Katie continues: "That is absolutely fantastic. Junior is going to love that."
Harvey replies: "Of course Mummy."
She adds: "When we give him his presents and his cake - what do you think?"
A clapping Harvey yells with delight: "Oh yeaaah!"
The former glamour model captioned the heartwarming video: "How adorable!
"Harvey has recorded himself playing the keyboard and drawn this picture for his brother’s @officialjunior_andre birthday today."
Fans rushed to congratulate Harvey on his artistic efforts.
One user gushed: "When it finished and you said 'that's absolutely fantastic'. You could see his smile in his eyes because he knows you're so proud of him."
A second user praised: "That's amazing Harvey x Kate you do a brilliant job x he really is a lovely lad."
And a third posted: "He is so lovely. What a lovely thing to do for his brother."
A fourth congratulated: "How clever is Harvey - the 3D detail wow he’s super clever you are so talented and creative Harvey well done xx."
"Harvey well done xx"
Meanwhile Katie paid tribute to her second eldest son by posting a collage of photos of Junior.
The montage includes Junior as a baby, toddler and with all his brothers and sisters.
Katie captioned the sweet video with: "Memories are the best. Happy Birthday 15th Birthday to my @officialjunior_andre."
His birthday comes just weeks after Katie celebrated her 42nd with all five of her children visiting her during lockdown.
What do you think of Harvey's picture? Share your thoughts with us on our Facebook page @EntertainmentDailyFix.