why has my chihuahua taken a dump on my sons bed?

I think what it sounds like to me is that your dog is jealous of your son or mad at him about something. I have a dachshund that will poop on things that belong to my fiance when he comes home, but when he is gone he never once poops on anything! I believe it is pure jealously! Sometimes if you give someone or something more attention than you normally give your dog they do things like this just to try to prove a point! They aren't dumb animals!!

Because the dog is not being properly "potty trained".

Dogs don't discriminate when it comes to where they will hunch over and take a poop, they will do it on someones face in there sleep or on another pet's bed or on an expensive device left on the floor if given the chance, or if possible, they have to poop and that's all that they care about doing, they don't care about the place.

First of all, stop your dog from sleeping with your son if he is doing this. That is absolutely gross, and I'm sorry your son has to deal with this.

Every 2 HOURS, take the dog outside to use the bathroom. If and when the dog uses the bathroom, pet him and give him a treat, to show him he's pooped somewhere positive, repeat this daily, and regularly, until the dog gets it, and THEN and only THEN would I let that dog even think about sleeping in my child's bed.

Edit: Well if you would have listed that before that could have been a key factor and would have helped me give a better answer. Some dogs poop to scent mark, or do to changes in their environment. Your sons "girl-friends" scent could have triggered all the changed your dog needed to act different and "scent mark" to cover up what he considers to be a "foreign" intruder in his territory, "the girls scent".

I suggest you not let the girl sit on your sons bed or come anywhere near it, as small dogs are more prone to act differently or territorial to environment and scent changes then larger breed dogs.

They do it to "Mark" their territory just as urinating on beds does. Is there cats or any other dogs in your home? Sometimes they just do that to claim that as theirs. The only way to stop is is close the door.

Rebellion.