The idea of an "intervention" meeting with the adult siblings is, I think, a very good one. The boy apparently believes no one knows he has been missing school. I frankly am appalled that the school waited nearly twenty days -- twenty days! -- to call anyone.
At the family meeting, if the tone is accusatory and harsh, the boy will get instantly defensive and clam up and say nothing, or yell, or leave. Your husband needs to set the tone with the siblings and tell them in advance that they are there for support but he, your husband, will take the lead. And he should write out a script for himself -- seriously. If he talks off the cuff he may get angrier and/or more upset and things could degenerate into argument. A statement about his worry and fear would help; do you think the boy realizes how terrifyign it is for adults to wonder: What has he been doing all this time? And your husband needs to have a list of What Happens Next, which includes immediate return to school, whatever consequences the school has in store, and a consequence from the parents, such as grounding that MUST be enforceable so he is not sneaking out.
The adults have to listen, too. Has the kid been failing at school and he left out of fear of more failure? Has the kid been undergoing any bullying or other issues in school that made him leave? What has he been doing in the hours that his parents thought he was in school? Does he realize that now you have to ask whether he is doing or dealing drugs, who is he seeing during that time, is he in trouble with the law?
I would use his pride in his confirmation to emphasize that God wants children to respect their parents. And his parents now know he was not respecting them. If it would help -- is there a priest or pastor he really likes and respects? That person may be able to talk to him in ways the family cannot, or he may be truly sorry if he hears from that person how he has upset everyone. If he has a priest that he respects and likes, get that person involved to find out what he has been doing.
Reserve decison about the confirmation celebration until after the talk.
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