I would call your vet - it could be just a cough or it may be something your baby picked up there getting his shots - but it's always best to check it out -
Problem with coughs is once you get to the vet - they usually won't do it ! I took Chanel in for a cough and she was running around trying to kiss everyone in the office and acted totally healthy ! I felt silly but the vet understood - I never knew what caused her to be coughing
If you REALLY are wanting your vet to hear the sound - tape record the cough and take it with you ....GOOD LUCK with your little boy - hope he feels better