The 20-year-old has multiple disabilities, including partial blindness, autism and Prader-Willi syndrome, which can cause extreme appetite and weight gain.
Price opened up on Instagram as she shared a video of Harvey wearing bunny ears and eating carrots.
Although she mentioned the seriousness of her son’s weight, she also revealed how pleased she was that he had lost a stone.
The star also praised the staff at her son's residential college.
“I’m so proud of Harvey he has lost a stone now and his college are doing amazing trying to keep him active and healthy eating as it’s now become life threatening with his weight,” wrote the mum of five.
Price - who had Harvey with former flame Dwight Yorke - added the hashtag “#lovemyson”.
The model and TV star has previously opened up about the challenges of raising Harvey and last year told how she made the decision to put him into care at a residential college after reaching “crisis point”.
Two years ago she told The Sun his weight had reached nearly 29 stone.
“It’s a cruel and horrible illness where he feels hungry all the time - and if he doesn’t get food he gets angry,” she said at the time.
“It’s hard work. He’ll smash things up if you say no. I’ve had to re-plaster my walls because he’ll bash holes in them with his head.”
Price is also mum to son Junior, 17, and daughter Princess, 15, from her former relationship with Peter Andre, and son Jett, nine, and daughter Bunny, eight, with her ex Kieran Hayler.
Watch: Katie Price reveals she was raped at gunpoint by car jackers in South Africa