Hi Nicole! Your little birthday boy is going to love having breakfast with the Disney characters!
I definitely understand not wanting to be over-scheduled when traveling with young children who might get tired and cranky if they don't have time to rest. During most times of year, you can usually find a restaurant in which to have dinner on a moment's notice, although it may not be one of your top choice restaurants. Just call the Dining line from your resort room or check the
Dining Reservations page of the Disney website to see what restaurants have availability on that night.
I notice that you're visiting over the Easter holiday, which is a period of peak attendance levels in the Disney parks, and for this reason, getting into a restaurant without a reservation may be more difficult than at other times of year. Quick-service restaurants will always be an option and you can likely find openings at table-service restaurants, but they may be at much later times that you'll want to dine with a little one. If there are certain restaurants that you think your son would really enjoy for a birthday dinner, consider making a reservation at one that doesn't require a credit-card guarantee, and then if he's not in the mood to sit for a meal, you can cancel it at the last minute. That way, you'll have a plan in place and won't be scrambling at the last minute.
Wishing your son a magical birthday! I'd love to help you with more questions -- just come visit me again at the Moms Panel!