Less than a week away before we leave for the camps, saw some discouraging news this morning though. We have tickets for the SF / Cin game the night of the 27th, checked weather and the low is expected to be around 55? wtf its almost July for gods sake. Last time i experienced anything below 70 was more than 10 years ago. lol
Dad,
AT&T is one beautiful park. We celebrated Father's Day there and it is a favorite place for us.
The Summer weather won't be confused with most of Hawaii, though. You might compare it with being on Lanai, near Lanai City. Usually nice during the day but the fog comes in around 4-5pm and the cool down begins.
The extent to which layering is needed at AT&T depends on seat location, in our experience. In the bleachers or along right field...layer.
If you are in the stands, any fog/wind will blow above you unless you are in the top level, near the top. Watching a game from along 1B or 3B in the field level is just great and the weather with the crowd will be warmer than you are reading.
My guess is you and your son will also want to walk around the entire stadium so a fleece is usually very comfortable and all you need. Also, you will want to check Willie Mays Plaza and his statue, McCovey cove beyond RF and his statue and the Marichal statue just outside the stadium near the Giant's dugout store on 3rd Street.
You can take the train from Palo Alto and it drops you off one block from the park. The challenge is there is only one train after the game and it stops everywhere so the ride home to Palo Alto can take close to one hour.
Infield,
Celabrated fathers day at AT&T as well. Great weather, couple of long balls, terrible out come for a Giants fan.
Kauai,
There is plenty to do and see around the city. We were at thepark for a day game and the temp was 65, but felt like 75-80 in the sun. As recommended , layer up. PO jr did Stanford camp summer after junior year and loved it. Good luck!