How sweet is it that your kids want to plan a birthday surprise for dear old dad? I love that. You've given me quite a challenge, though, considering you're looking to dine on a budget
and you've already visited most of the park/resort restaurants. I'll give it my best shot, though. Deal?
My first suggestion would be
Raglan Road Irish Pub and Restaurant at Downtown Disney. It's a lively place, perfect for celebrations. The birthday boy might enjoy watching the Irish dancers and tapping his feet along to the live music that is played intermittently. The fun and festive atmosphere is sure to make his day a happy one. And with regard to cost, Raglan Road is relatively inexpensive (by Disney standards), with most entrees falling in the $17-$25 range.
For a more reserved dining experience, you might consider dinner at
House of Blues (also located at Downtown Disney). Even if you've been there before,
recent menu changes might make this a spot worth trying again. Enjoy!