Will be in San Fran June 2 through 5. How should we pack? What kind of weather should we expect?(40 something parents and 21 year old son)

One word: layers. You may have a hot day or two, you may have a foggy, chilly day or two. I'm sure you already know to check weather.com a day or two before you go, but, sad to say, you need to be prepared for big fluctuations in temps.

Expect temps from low 50's to high 70's.
SF always feels abit cool to me (but then I live in Phoenix), but you'll need a variety of clothing, not just typical shorts/shirt summer attire.

Definitely follow E's advice about layers. Often it will be foggy until late morning or early afternoon, when the sun comes out for a few hours. Of course it depends on what pert of town you are in.

Devi:

"The coldest Summer I ever spent was a Winter I spent in San Francisco."

Sam C

You don't want to be one of those poor souls down at Fisherman's Wharf in shorts shivering your a** off! They do a bang-up sweatshirt business there. Definitely dress in layers, wear your walking shoes and be prepared for all manner of weather, except snow, of course. If you are leaving the city at all to go to the wine country or other spots it will be warm and sunny. Well, the coast might be suffering from June gloom. Layers are the key!

In drag!!

Devi,
Do not .... and I repeat, do not wear shorts! Even on the warmest day in S.F., a pair of chinos is fine. Remember that the breeze off the Bay can come up at any time and can make even a sunny day feel a bit chilly. All of the above posts have gotten it right with the advice on layers. Take a jacket with enough room for a sweater underneath.
But don't be afraid of the fog here. It's a force of nature, majestic when it sweeps in over Twin Peaks or through the Gate, puzzling when it seems to follow only one little patch on the bay, and always moody, mysterious and, hey, it's good for your skin!

Fortunately your son is old enough to begin listening to you again. We were stupid tourists freezing half to death in July in SF. It was hot on one day for about 2 hours - the rest of the time we wore long pants and sweaters. On 7/4 we had on all the clothes we had with us and still froze watching the fireworks - it was warmer in Seattle the week before.

Remember being invited to the old Candlestick park one July day as a guest of a 3 sides enclosed box to watch a baseball game.
Even in the enclosed box, there were both floor and ceiling heaters.
Trust the previous posters; dress in layers.
San Francisco isn't really a 'shorts and T-shirt' kind of city anyway.