I think your son giving you away would be perfect. I bet your dad and your son both hold a very special place in your heart so instead of thinking of it as "replacing" your dad, think of it as finding a fitting person to temporarily stand in his place.

Also, I saw on a wedding show that a couple whose fathers had both passed paid tribute to them by having their wedding rings (or other jewelry) melted down and made into their own wedding bands.

Congratulations on your upcoming wedding. Best Wishes to you and your new family.