Cannabis fumes fill my sons bedroom from next door and build up when the door is closed what effect will it have on him?

Not good: You should not let him smoke in your house. I don't believe that marijuana is legal in the uk. I might be wrong on that however. If it is illegal then it should not happen anywhere near your house. Furthermore, he is accumulating higher and higher blood levels due to secondhand smoke which can have the same effect as cigarette smoke. It is simply not healthy for him and in the long run he can get pulmonary cancer. Psychiatric risks are psychosis, depersonalization disorder, and social anxiety disorder.Since it is coming from next-door, i think you have no choice but to talk to the people next-door and if it is illegal turn them into the police. You should not have your son's health endangered by their behavior.

2nd hand smoke: Your son is being exposed to second hand smoke at the least. If the smoke and fumes are strong enough, he could possibly experience some second-hand intoxication from marijuana as well. Chronically breathing smoke from whatever source is not healthy for his lungs or for him.