If he has already been stung and is waiting on answers from here the worst will be over. There is really no antivenum. The severity depends on the type of stingray. Most will give a pain like a very bad bee sting. Most common and best first aid for one is hot water on the sting, it breaks down the protien (i think that is what it is) faster and gets it over with faster. After that you just need to keep it clean so it doesn't get infected.