Rat sputniks make good hammock equivalents
But if you're looking for crafts to make then that doesn't help!
Someone on here made some very nice tunnels by wrapping (glueing) sisal string round the outside of pringles tubes
Also your son might like some of the ideas on this video. I've made the parcel and a sort of version of the hammock from this (but I used an egg box carton after seeing the idea on a link of Vectis's). Sisal string and thicker sisal rope are very handy to have around.