I echo what reeyah and DC17C say - it is very normal, especially now that the 2012 candidates have started talking more openly about the exam as it approaches. It is a very daunting thought for some of them, they start to realise that there is a chance that they might not even 'pass' and separation from friends may be a difficult idea for them. Although my eldest two sons were often meeting up with their friends after school, I doubt either of them would have been upset for long at the idea of parting with their friends earlier on in primary school, it was only at the end of Year 4 and Year 5 that they built stronger bonds with them. So it is natural that they might not want to part with them now.
DS1 is the only one from his primary school going to his secondary school and it is daunting: the only thing I keep telling him is that as the friends live locally, he can still keep in touch with them. Besides, I remember I insisted to go to the local comp with my friends, instead of the indie my father was prepared to pay for............... guess what? I was put in a different class from most of them, ended up in higher sets than the ones I was in class with and couldn't stand any of them by the end of the year anyway!
Hopefully, it will pass. Give him a week off work, let him enjoy the sun while it is out, maybe plan an outing at the weekend. I hit a big bump with DS1 2 months before the exam and followed this advice - it took a couple of weeks, but he did come round in the end.