Hot water is the general treatment for marine stings. Do that for the short term, but I'd at least call the doctor tomorrow, someone who knows his medical history and potential complications (it'll send him scrambling for the manuals.) I'd go straight to the ER if there are any signs of sweats, red lines, high pulse, etc. They can give medications to stop a reaction, if it should occur. If he has a chancy medical history I'd get there just as a precaution...again, probably none of these things will happen. I've been stung by a brown recluse spider, and it did help to get a steroid shot right into the bite area...but nowadays they tend to hand you a Medrol dose pack and shove you right back out the door, at least for that situation: my point being that for bites and stings sometimes getting quick, aggressive treatment can make the healing much faster.