No idea why coke would help the pains Debra. Interestingly enough my first job as a Dr was on a paediatric infections ward and we looked after a lot of gastroenteritis as part of that. The children hardly ever needed a drip putting up, but the nurses were probably just a little bit tougher with the fluids only regime than the parents, as of course there was no emotional attachment to get in the way. Anyway, we used to use peppermint water (prepared in the pharmacy) as treatment for the griping pains, and it worked really well. In fact, peppermint oil capsules are still used to treat the pain of irritable bowel syndrome, which is also of a griping, colicky nature, so peppermint water is highly recommended if you're stuck (and if you can get hold of it from a pharmacy - not sure how easy that is in France).
The electrolyte sachets (Dioralyte, Rehidrat, etc) are also very useful for younger children as gastroenteritis can affect the electrolyte balance in this young age group in severe cases. Water is probably ok for 24-48 hours then the sachets might be needed. That being said, if you've already got some you can kick off straight away with them, though they have a slightly salty taste so sometimes aren't well tolerated (when I was in the said job above, for a joke one of the nurses made me a cup of tea made with water and a plain dioralyte sachet, I nearly threw up myself it was so foul!). Smecta is a more modern version of Kaolin, a clay-based product which used to be combined with minute amounts of morphine. I don't know if Smecta is very effective, and certainly using clay or similar products went out with the ark in the UK. Having said that, it is a more natural way of treating the diarrhoea than loperamide, which just stops the bowel in its tracks. Pleased your little fella has fully recovered now, its amazing how quickly kids bounce back to full health, but thankfully they usually do!