If you have been immunised against whooping cough you apparently don't make the whoop sound anyway I was told when I had it and just coughed. The only way they can diagnose whooping cough is with a throat swab or blood test, DS had to have it 2 weeks ago cause the GP thought he had whooping cough (he just had a really barking cough and a fever)
I'd take him to see a GP today.