If the cat is still young you might be able to train it to use a litter tray. Where do you expect it to go if it is not allowed to go in your garden? It has to go somewhere, and it is/was you decision to keep a cat so it is your responsibility to provide it with suitable toilet facilities.
Probably not what you want to hear, but there you are...
Try putting a pile of litter or the tray where you would like him to poo,perhaps at the edges of your garden near the field or in the borders ,and show him. I think it's always easier to get a cat to do something rather than stop him from doing something!
And take the existing litter tray away if it's indoors -he obviously would rather go outside anyway.By the way,are you sure it's him doing it on your lawn.?
lots of orange peel put down anywhere where you don't want cats to use as a toilet works brilliantly, keep reapplying it though. Put some of cats dirty (lumpy) cat litter in the garden where you would prefer him to go. I have to rake the soil in the winter for kittens, to soften it up, so they can scratch properly at it and then cover it up sucessfully! Good luck