Hi Ana,
How terribly sad for the boy, and also for you and the rest of the class.
I �ve never been in this kind of situation so I don �t really know what to suggest, but I saw that noone had replied to your post yet (probably because it is such a difficult situation) so wanted to say something to you at least.
I would say go with your instincts. You know the boy and the rest of the class and what they need the best. So do what feels right.
Depending on the age and personality of the class, I feel it may be helpful to talk with the other classmates before the boy comes back about what has happend and ask them to be sensitive. Maybe make lists together of things you could all do to help him feel that he is surrounded by good, caring and loving friends/teachers. I think that is the most important thing for you all to do - to show him the love you all feel for him, to be there for him when he needs it, to give him space when he needs it, to encourage him but not push him to continue with his life despite the pain and sadness.
As I don �t know the inidivual involved it �s rather difficult to say, but my instinct would be not to make too much of a big thing about it when he gets back. Of course you as the teacher should say how the whole class feels for his loss and let him know that if he needs any extra help/support/a shoulder to cry on etc. you will be there for him, but I kind of imagine that getting back into a familiar routine will be quite consoling for him, so I think that other than a brief talk with him (not sure but probably privately is best) you should just go about your class routines as normal, obviously watching out to see how he is doing.
Is there a counsellor affiliated to the school he can talk to? Or a close adult friend/teacher? If so, it may help him to talk through things with them when he is ready.
I guess what i really want to say is let him heal in his own time and show him all the time how much you and the rest of the class love and care for him. And be patient if he becomes difficult, but don �t turn a blind eye if he turns impossible.
Is this any help to you? I do so hope so.
Good luck with everything, I �ll be thinking of you.
A big hug
Julia