Hi, Craig, Your brother-in-law is very lucky to be celebrating his birthday at Disney! If he will be at Disney World on his actual birthday, have him register
www.disneyworld.com so he can get free admission to the park on his birthday. Also, have him stop at guest services to pick-up his "Happy Birthday" button.
You can turn any meal with your family into a nice, low-key celebration! When you check-in for a meal, ask at the podium if you can order a cake to have sent to your table. Or, call the Cake Hotline ahead of time at 407-827-2253 to have a special cake delivered to your resort room or to a special restaurant, or whatever you decide.
I think if you allow your brother-in-law to pick the restaurant for his birthday celebration, you have a cake, and he has his family with him, this might be a great way to celebrate! (Make sure you have dining reservations - call 407-WDW-DINE (407-934-3463.) Have a great time!