At work the other day we were talking about new research (I was working in special care nursery) and it showed that in fact a lot of babies have milk in their lungs when biopsies are done. It showed that there must be alone sort of natural ability for bubs to be able handle some aspirated. Interesting. I would watch your son closely and if he looks like he is having to work harder to breath or coughing a lot I'd get him checked out.