We had this with our neutered cat peeing in DD1s car seat and a toy basket. It's horrible and a right pain to deal with, you have my sympathy. There's a trigger spray product you can buy in the pet shop called "Get Off," safe to use around DC, which will deter your cat from entering the room if you spray the threshold. You also need to make sure that absolutely no smell of the previous incidents remains as that will encourage him to try again. So maybe take your DS's duvet to the launderette and use a large capacity machine to give it a really thorough wash if you think there's the slightest chance your domestic washing machine might not have done the job. You could also try removing the duvet/top layer of bedding during the day, so if he does get in there's nothing to scuff up and your DS's bed is less appealing as a litter try alternative. Territorial peeing in a cat with no previous history of this kind of behaviour might suggest anxiety, so is there a chance he's been fighting with other cats, or has had some kind of bad experience recently? It can lead to (or may mean he already has) a UTI as the cat will strain to produce urine and this can irritate the bladder. So for his sake, as well as your sanity this needs sorting! Good luck.
Sadly I have lots of experience of one of my little darlings weeing in inappropriate places. I replied to another thread recently with a lot of tips - take a look at this one
Also, when washing anything in the machine, make sure you use biological washing powder as it breaks down the enzymes in the urine, thus removing the scent from sensitive feline noses more effectively....
well we have a new baby and he does kep trying to get into the moses basket for sleep, so perhaps hes feeling a bit insecure or something. he has actually peed on the mattress this time and i just cannot shift the smell, i'll have to bin the mattress and get a new one. ds is in my bed tonight.