normally they aren't an issue but since it is causing pain I would have a second opinion. My daughter was born with one on the back of her neck and she is now 20 months and it is slowly dissappearing..
No, you should not continue just to leave it.
Although birthmark hemangiomas can go on their on from around the age of 5 to 10, your son's has got bigger and darker and hit hurts. I am surprised you were not advised that should it change and start to hurt that should seek medical help straight away.
You must not ignore this.
You need to get him to see the doctor as soon as possible. The doctor will be able to assess him and decide the best course of action.
Best wishes
If it is painful, and there is discoloration, you need a consult with a dermatologist. This should be a priority.