This sounds cruel, but it worked for one of my cats, Jarvis (now long since passed) - I gave him food I knew he didn't really like.
![Evil Laugh :evillaugh:](https://www.purrsinourhearts.co.uk/Smileys/purrs smilies/evillaugh.gif)
He was a Go-Cat cat through and through and wouldn't touch anything else.
I simply switched him to Applaws which he ate grudgingly, and presto, the weight dropped off. As soon as Jarvis started to lose weight (and gain condition by the way!) his energy levels rose and he would spend all day playing out instead of lazing about in bed.
![Excited :yesss:](https://www.purrsinourhearts.co.uk/Smileys/purrs smilies/202.gif)
Some cats might find a sudden change of diet makes them sick, so this approach is very much to be used with caution.