They are known to be chameleon safe, however, the milky sap that drips out when you pull the live leaves can irritate the skin and eyes of a Cham, I don't think it's really dangerous though. Just keep an eye out for any problems and be ready to correct them. As for him ingesting it, he should be fine, however, I personally would be concerned if he was really eating a large volume of it.
No, they're not if eaten! Ficus sp. contain furcocumarines and flavonoid compounds. I know cases with chameleons eating Ficus and getting in trouble due to that (vomiting, diarrhea and one even got paralysis after regularly eating huge amounts of Ficus tree leaves). Nobody knows whether a chameleon needs to eat huge amounts to become sick or whether there are individual sensibilities. Therefore chameleons should not eat more than a leaf or two occasionally, otherwise you need to replace the plant. You can try to offer eatable, non-toxic plants as Hibiscus, Calystegia or Tropaeolum before to see whether he prefers eating those. They're all easy to grow at home/inside cages and got nice coloured flowers, which seem to be very attractive to our "plant-eater-chameleons".
Eating plants is even no definite sign of dehydration. There are lots of well hydrated Chamaeleo calyptratus nibbling plants, some more, some less.