I have used the Cardiff National Poisons Unit for major incidents, and you may find they are willing to help, or you may have to go through your GPs surgery.
The standard work is Poisonous Plants in Britain: and their effects on Animals and Man, publ HMSO ISBN 0 11 242529 1.
A quick check in my copy says Clematis (Old Man's Beard) All parts are said to be poisonous, and sap can cause blistering. Gastro-intestinal irritiation and severe abdominal pain.
Clematis is poisonous but I do not know what quantities are required to be toxic to a child or adult if eaten.
So I do Hope you contacted N.H.S Direct or your GP for advice, even if symptoms are not immediately present it could take a little while before they do (if at all hopefully) depending on his age and size and how much he has eaten they will advise on any 'First Aid' that may be required and the best course of action either way.
It's been a little while since your posting I hope your son is ok.
Hopefully your anxiety will have passed by now, but if your youngster is seriously affected by eating Clematis I would be seriously surprised.
There are a few plants that you would try to avoid in areas where youngsters are going to be, for example yew and laburnum, but I have never heard of problems with Clematis.
Can be fatal to cattle; last known death in UK was 1968, and farmers are generally well aware of keeping stock away, even though it is not preferentially grazed.
International Veterinary Information Service states
"Death may occur in children eating large amounts of the plant or berries." The full citation is not presently available.
I sent PMs to csail twice this afternoon but no reply.
I think cattle are more likely to try and eat serious quantities than a youngster. If you look up 'poisonous plants' you will see that daffodils, for example, are also included. I've seen one list that includes apple seeds - if you eat enough.
Obviously the quantity eaten can range from a few grams to a few thousand grams and therein is a huge difference.
Let's hope csail and his youngster are laughing about it now - and that they let us know how it turned out.
Our local garden centre is selling bags of daffs, and there's a great big warning telling people not to eat them. Rhubarb, foxglove, horse chestnuts, buttercup, oak, laurel, lily of the valley, you name it, someone has tried it and not won.
Yes, I echo the concerns about csail and his son. Fingers crossed.
sorry for late reply...he is fine. Swmbo is a doctor and by coincidence her lab is next to toxy in Llandough Hospital, Cardiff! Just she was in a meeting at the time.
A friend of mine accidentally ate daffodil bulbs, believing them to be onions. He's been in hospital for a while but is expected to come out in the spring.
He did well to find it. According to my wife I couldn't find a clematis (I think that's what she said...) with a map & torch /forums/images/graemlins/wink.gif
Glad your son's OK - must have been quite a scare.