I can't offer you personal medical advice (no one can over the internet), but I can address your frustration.
None of the medical staff I have taken him to is taking this seriously. How much longer does he have to be having diarrhea before they finally take it more seriously and do something?!?!?!
I think this is the gist of your question. For this, I have a couple of suggestions. First a bit of background.
Doctors see diarrhea all the time, and it is - fortunately or unfortunately - less concerning to them, because, again, they see it all the time, and by a large margin, the cause is relatively benign and the children get better mostly without much in the way of medical treatment (home treatment often suffices) or with a course of antibiotics if cultures come back positive. It might help you to know that:
The most common cause of diarrhea in kids is infectious: viral more often than bacterial which is more common (depending on your location) than parasites. If they have attempted to rule out the treatable and non-self-limiting infectious causes (with appropriate cultures), that's the right thing to do. Suggesting probiotics is already being ahead of most.
There are three degrees of severity (with overlap) of diarrhea: mild (2-4 watery BMs per day), moderate (~4-8 watery BMs/day) and severe (~10 or more watery BMs/day.) Along with number of bowel movements, one needs to take into account amount; explosive diarrhea is more serious - that is, it usually leads to more fluid loss - than "regular" diarrhea. Dehydration is the main concern for all diarrhea; if the child is well-hydrated on repeat physical exams (or by parental history on questioning), the main danger is avoided.
If the child is well otherwise (that is, 98% of the time: he plays, eats, runs around, has normal amounts of urine, sleeps through the night, has a normal temperature, etc.), and only complains in the few minutes before having a BM (it's normal to feel crampy), to physicians that is a "reassuring sign", though it might not be to you. (As a parent, I worried much more about my children's illnesses - especially under one year of age - than their doctor(s) did, even though I had treated many children with the same things capably and confidently.)
Diarrhea in younger people - infants and preschoolers - usually lasts longer than in older people because of intestinal factors. Two weeks of diarrhea in a 2 year old without any other signs or symptoms is usually not even a worrisome thing to most doctors.
These are some of the reasons your doctors might not appear to be taking this seriously. It doesn't mean they don't care. In all likelihood, they do care, and they are - in all likelihood - doing what many doctors would do in your situation.
Things which will make doctors (and should make you) more concerned:
- diarrhea of greater than two weeks duration (which requires that cultures be taken)
- Bloody diarrhea
- lethargy, unusual behavior
- abdominal pain
- weight loss
You are doing all that you should be doing. You have been keeping good track of his bowel movements. He seems in good condition other than the diarrhea. Take his temperature several times/day, and watch for bright red streaks of blood in the BMs. Keeping a food diary is never a bad idea.
Read up on causes of diarrhea in kids. The more you know, the better you can address your concerns with your son's doctors. Asking informed questions (e.g. "Is diarrhea at this age and to this degree dangerous? Can this be non-infectious? How long do you wait before deciding to do a work-up?") is more likely to be answered in a manner you find helpful than just worrying that they're missing something.
You can't force your doctors to act more concerned than they are, but you can ask them questions, which they should take time to answer. If you don't have confidence in your doctor(s), it's always appropriate to seek another opinion. Also, the ER is there for you and your son; they cannot (in the US) refuse to see your son. (A professional secret - in the US, at any rate - if there are multiple ER visits, you'll likely get to see a specialist sooner.)
This site is one of my favorites for general problems. There is a list of the tests doctors should do in your son's case (routine cultures, Ova and Parasites, Rotavirus antigen, Giardia antigen, Hemocult test, and any that might be specific to your area); you can make sure they have tested for all of these.
If after reading the article, you still feel that the doctors aren't taking things seriously enough, please take your son to another pediatrician or the ER. In many scenarios, it's the squeaky wheel that gets the grease. Never be afraid to be a squeaky wheel if you're genuinely concerned. Good doctors would rather have patients who tell them of their concerns (so that they can meet the parent's expectations and gain their trust) than that a patient hold back for fear of being a "bad patient".
In this article, a mom who had doubtful physicians stated
[Parents] have to advocate for their children, because if we as parents don't, no one else will, and advocating for your child, standing up for your child, pushing for the right diagnosis and treatment for your child doesn't make you a nuisance. It makes you a good and loving parent.
This one about advocating for your child states
...any reasonable provider should never try to block parents seeking second opinions...
Granted these were rare illnesses. Diarrhea is not. But the principles are the same: educate yourself and keep asking questions. When in doubt, seek a second (or third or fourth if it continues) opinion.
Diarrhea in Children