Make sure it is the croup. Ds has a condition called trachiomylashia. It is sorta like asthma of the trachia. It produces a cough that sounds much like the croup. It did not start till he was like 12. It took a pediatric pulminologist to diagnose. They thought croup, whooping cough etc. He was premature and had a vent that caused damage but that is not always the case!
Just a thought.
I'm quoting this post just to emphasize that "croup" really just describes a symptom, and not a disease. If DS is still getting a croupy cough, it's time to explore the root causes a bit more. The condition quoted above is one possibility. Another possibility is what we deal with with DD7. She has eosiniphilia esophagitis - luckily, a fairly mild case. Basically, there are cells in her throat just past her vocal chords that are extremely acid-sensitive. When she has a cold, and post-nasal drip comes in contact with those cells, they become inflamed and swell. The swelling causes a croup-like bark-and-strider cough. We finally "discovered" her diagnosis after dealing with major croup episodes about every three-to-five weeks for about a year, starting when she was about eighteen months old.
Her issues are now pretty well controlled with a daily dose of an acid-suppressant medication (she started taking Zantac75 pills at 18 months, and has recently switched to generic Prilosec or generic Tagamet, as the Zantac appeared to be losing effectiveness). When she gets a croupy cough anyway, she does a one-week cycle of Prednisone.
Diagnosis can be difficult, because even many pediatric ENT's aren't well-versed in "EOS" yet (more serious forms of eosiniphilia have some kids on a liquid "basic" diet). We were lucky to get a diagnosis - we had a scope of her throat done at a Children's hospital, and they totally missed it. But we sent the pictures from the scope to a friend of the family (my BiL happened to be dating a well-respected pediatric ENT), and she found the eosiniphils immediately. There are good EOS docs at the Mayo Clinic in Rochester, MN and at the children's hospital in Cincinnati, OH. I'm sure there are others, but those are the good ones that I know of.
In the meantime, you might try cutting down or out on highly acidic foods, like tomatoes and orange juice.