Here's my experience on the topic:
My wife and I had a 25k+ wedding. Neither of us wanted it (we wanted to elope), but it was important for her mom. To keep family peace, we did it. She made it clear what she would pay for, and we gave her 'control' to plan those things accordingly. Everything else that was on us, we planned (and kept on a nice budget). If she didn't like something we planned, she would contribute towards that.
It may sound ridiculous to some, especially to mustachians, but it worked for us--and the peace that was kept was definitely worth it.
My parents, on the other hand, made it very clear the specific amount they would contribute. They didn't care where it went. Rehearsal dinner, honeymoon, dress, flowers...didn't matter. That's how much we got. And that's how much they'll give ALL my siblings. They just wanted to be fair.
Two different methods, peace was kept on both sides of the family. Our relationship with either of our parents took no hit, and has since only gotten better (THIS IS THE ULTIMATE GOAL.) If a big wedding is important to your son and future-daughter, then don't stop them. Just be honest with what you're willing to 'chip in', they can figure it out from there.
Good luck!