My nine month old son is having watery stools with tiny white chunks in it. What could be causing this and should I take him to the er?

? yeast infection. Candida albicans is white (hence name} and i suspect that is the cause of the diarrhea. Look up great plains laboratory for an easy way of proving. They have special requisitions for a urine test. It must be signed by your pediatrician. If it proves yeast, there is treatment available.Get it checked out.
Diarrhea . Diarrhea in an infant can get serious quickly. Usually, it's due to a viral infection; the problem is that they can get very dehydrated very quickly. If he can take fluids by mouth, give him pedialyte & try to see his dr asap; if he starts throwing up, go to the er.