My son is upset because his wife of 13 years told him she and her ex-husband will be walking her daughter (2nd marriage for daughter) down the aisle. Is this appropriate?

Its appropriate if that's what the bride wants. It may hurt some feelings, but she loves her parents and wants them to be a part of her special moment.

I don't understand why he's upset, frankly.

Was he expecting to be the one to walk her down the aisle?

No reason for your son to be upset. They are participating as her parents, not as a couple. They are there because of their relationship with her as their daughter, not their relationship with one another.

After they walk her in, they should each sit with their own current spouse. And, the bride will leave the church with her new husband, so your son can walk out with his wife, and her ex-husband with his wife or guest.

I think it would be inappropriate for your son to make an issue of this at all.