I've had it done but then again I wasn't as young as your DS. And was probably testing for different things. with me they basically drew up a table on my arm into like squares and then put drops of different allergens, eg. grass, animals, dustmites etc. in the different squares then got this little needle and pr!cked each one. Couldn't feel that though. Then they covered it up and waited, then measured the results. I reacted quite badly to grass (it was seriously itchy!)but they just put some antihistamine cream on it. So what they will do for your DS might be a bit different, and at least they are testing him in an environment where they can intervene so I guess best to find out now then when he's at home IYKWIM.

though, it's always the unknown that causes the worrying. HOpe he goes ok!