I don't think the ratio "inner monologue versus sex scenes" is the problem really... I am gonna be blunt here, but really those monologues are waaaay too long and dragging, in the way that the thoughts of the character barely make any progress at all or trigger anything. When you have a dialogue scene between the son and the mother and the inner monologues of BOTH make up for 90% of the text you read, then you have a problem. It just slows downs everything, it becomes boring, also because the images for the scene are basically 3 different angles and expressions from the same face for 5 minutes! My advice, try to heavily reduce the length of them, keep it short and useful for the action and the developing story...
I kind of like the premise of your story, btw, the triangle, the darker plot with the doctor...I am just not sure what to think of the boy, at first he looks a bit lost and sweet, but now he seems more like a horny swearing a-hole who constantly calls his "mother"(in his mind) a bitch... He becomes less and less likeable in my opinion.