Did it start out really watery and is now more soupy? Or was it soupy the whole time? If watery, I'm no doc, but my ds had watery diarrheal viruses twice around that age, which were most likely rotavirus. Once lasted 5 days, once 7. Both were hell. W/the 7 day one, I kept thinking it was never going to end!!! But it did. Interestingly, even though Sears and such always said that once they get diarrhea that it can take weeks to really heal/go back to normal, w/my ds, it went from this sick pure water (rancid smelling) diarrhea to normal with no in-between. Anyway, all this to say that if he has viral diarrhea it will end!
My day care's policy is if it's more than 2 a day they stay home. But 2 or less, and I guess they figure they're not sick, that it's teething or something similar. Is your day care's policy absolutely no diarrhea? And can you ask them what their def of diarrhea is (like pure watery vs. soupy?) Maybe they'll him back if it's gone from watery to soupy? Maybe getting the doc to say whether or not they think it's viral and whether or not they think he's can go back to day care, w/a note saying he's ok?
As to getting laid off, etc--what size is your place of employment? If it's more than 50, and you've worked there for at least a year (total time, doesn't matter if there were breaks for mat leave or the like) you're covered under the Family and Medical Leave Act (FMLA), which includes caring for a sick family member. This means they cannot lay you off, or even reassign you to a lesser job (I'm sure about the first part, the 2nd is a little iffy, but I know that if they reassign you it has to be to a job at the same level). Depending on the way your company administers FMLA, what you may need to do is contact HR and formally log your sick days as FMLA and you may need a note from your ped, but it's worth it if you really need protection from getting laid off etc (which by the way sucks that they would do that to you regardless of the law!). If you want me to expand on this any more, let me know and I'll try to answer. I'm not a lawyer, I just know about this mainly from a) having to deal w/it for my mat leave and b) from being a supervisor.