Hey, C.--I know you've already got lots of responses, but I can't resist adding my thoughts.
The movie was fun and the scary parts lasted less than 10 seconds each (Muggles being afraid as Death Eaters cause various disasters around them; a girl being thrown into the air after touching a cursed necklace; the Sectumsempra spell making Malfoy bleed; the Inferi-zombies crawling out of the water to attack Harry and Dumbledore; and, of course, the Death Eaters attacking Dumbledore at the end). If your son has read the books, he will be familiar with all of these parts and themes and it will not be anything he is unprepared for.
I found it very interesting that in this film adaptation, even though there are a few very sad and intense parts, the overall feeling was far lighter than in other HP movies. I have spoken with some of my friends who are also big HP fans and they have said they are disappointed in the movie because it feels more like any other teen movie and less like a Harry Potter adventure. I disagree, as my whole family enjoyed it and we have all read and love the books, but there was a lot more amusing teen angst than in previous films. I think the comic relief of various love triangles makes the intensity of the film exactly appropriate for a family movie. There was a lot of flirting and joking around, and that lightened up the sense of urgency.
There was some alcohol use, but it was not central to the plot (student having butterbeer at the local pub; teachers drowning their sorrows briefly). The violence is mostly suggested and always has consequences; Malfoy stomps on Harry's nose but Luna fixes it; later, when Harry and Malfoy fight with spells and Harry hurts Malfoy, he is genuinely remorseful. The use of violence is always (with the exception of warding off the Inferi) considered an unethical, "Dark" option.
My husband and I took our children, ages 7 and 3, to the movie on opening day and I was worried we were being reckless. But because they were familiar with the storyline it was totally fine. I see HP stories as a classic good-versus-evil dilemma and we have talked a lot with the kids about how even when sad things happen in the stories, we know good will prevail. I think setting that tone diffused any potentially scary parts. (Also, I distracted my younger son with candy when the Inferi came out and he was oblivious.) There's one startle moment in the whole film.
If your son who is a HP fan has not yet listened to the audio books, I highly, *highly* recommend them. Jim Dale performs them with various voices and they are amazingly engaging and fun. My family enjoys the movies as an afterthought, but we dearly love the books, particularly listening to the audio books. They are positively magical for all of us, and might be a nice introduction to the series for your more sensitive son.
Best wishes!