Hi Santos!
You have picked a great time to go. The crowd levels are not usually as high in September because most children have returned to school. Sept. 6th is Labor Day, and Sept. 9th is a "Night of Joy" concert, so you can expect crowd levels to be a little bit higher than the other days. Still, they are not as high as most events or holidays.
The Quick Service Dining Plan option is great! You have lots of great dining locations to choose from; you can read about some of those choices
HERE.
Consider making an Advanced Dining Reservation for a special Character Dining experience, especially to celebrate your son's birthday. Check options and make your online reservations
HERE, or by calling (407) WDW-DINE. This will not be included on your Quick Service Dining Plan option.
One thing you can do for your son's birthday is to pick up a complimentary Celebration Recognition Button at one of the Disney Vacation Planning Booths. At Walt Disney World, we don't celebrate our birthday for just ONE day, we celebrate it throughout our entire stay!
Chip 'n Dale's Campfire Singalong at Fort Wilderness Campground and Resort is a free event You can enjoy the campfire at this special "meet and greet" with Chip 'n Dale. Cast members lead you in some singing around the fire, and you can also roast marshmallows for s'mores (s'mores kits are available there for purchase). Afterward, you can stay and hang out under the open sky to watch a Disney family movie outdoors! Talk about your "movie stars"!!!
What a great way to end a very memorable birthday!