In future a good one for inexperienced trippers is to tell them it's just the 'half hour willies', that every trip one takes they feel like that for about 1/2 an hour and just keep talking and keep them engaged. After 1/2 an hour they will have forgotten what it was they were freaking out about in the first place.. Though if it's just you and a couple of mates sitting around your house I wouldn't really advise that as the best place for 16 years olds to be taking powerful consciousness altering substances..
next time have some valium handy as an escape hatch. if the trip goes to shit take a double dose, and you will fall asleep
Weed will only increase the effects just tell him to calm down and enjoy the trip instead of fearing it