Why does my cat keep weeing on soft furnishings?

My cat (Oriental) is ten months old and was supposedly litter trained by his breeder. He does use the litter tray but he keeps on using beds, blankets and cushions instead. He is quite small, neutered, and has two bigger brothers. It is getting to the point where my mum wants to get rid of him and I really don't want that. Any advice on how to stop it/why he does it?