do you add chemicals? If so...what? the water should be OK, depending on how clean the pond is.

Even if he drank the entire pond the most he might get is a tummy ache. The fish are alive aren't they? They will die sooner than anything if there is something toxic there. Call 1-800-222-1222, they will send you some Mr. Yuk stickers, information, coloring books, all kinds of stuff. And they are available 24/7 everywhere in the US, even Puerto Rico. And it's free. Called the Poison Control Center. Your doctor usually will call them when you bring kids in for stuff like this. So they get free advice and YOU pay for it! You already pay for poison control, it's included in every phone bill in the US.
Tally HO!

I could see where there migh just might be a reaction to the water but doubtful I am sure that i have swallowed some water while cleaning the pond,I have been know to slip and go completly underto my dismay. The dogs drink it regularly and unless you have some weird parasite going on in the pond wellllll. Good thing is maybe your son will associate the tummy ach with pond water and wont drink it anymore. Let us know what it was please this is interesting

Hope this video can help!

http://californiawaterscapes.com/videos/vid-everyone_loves.html

Here is a link that might be useful: pond video

Most likely the child is reacting to the concerns of the adults around him but there are many things in the pond that are not good for people even if the water appears pristine. This subject comes up periodically at the Aquatic Plants and Ponds forum and while many get great pleasure from getting into the pond it really is not the best idea. This is mainly because it is a closed system that doesn't operate like a natural stream. What is in the pond usually stays in the pond unless we do something to eliminate it. Think of all the purification done before the water comes out of the tap in your kitchen. Think of all the chlorine in swimming pools before it is considered safe for humans to swim. An open wound like a scratch is easily infected by contact with pond water. Remember that unless you are a fanatic about filtration and aeration the pond is where the fish poop. I would be very interested in hearing what the doctor said.
The link below is a great introduction to the invisible things that live in the pond. Sandy

Here is a link that might be useful: Ron's pond scum

Nice to see people are still handing out Ron's URL :)