Does sound like the gastro we had a few months ago. Lasted for 11 days for DS. I struggled to keep those fluids in though as he was just pooing constantly so we ended up in hosp with a NGT. I was told later that there's a strain about ATM that remains in the gut longer than usual and is contagious for longer than usual (instead of the standard 48 hrs after last symptoms, apparently it can still be passed on 5-7 days later). Boy, did I feel terrible to learn that... we passed it on to friends of ours and my Mum.

I think once it's been going on that long though, it might be worth getting him checked out on the off chance it is something else. Did the rest of the family get it?