you really really need to talk to the parents about this first.
I am very against the people doing some kind of "show and tell" using DS's adoption. It is a private matter and IMO should be treated the same way as a child joining a family by birth not as anything unusual or special. It is not anyone else's business.
The family may not plan to be open with strangers about one of their childrne being adopted and announcing it to 30 5/6 year olds kinda blows that out of the water.
Also what a 5/6 year old can grasp about the concept of adoption is pretty limited. DS is nearly 6 and he just about understands that he was born fom another womans tummy not mine. I mean really understands not just knows IYSWIM. And he lives with the reality of it 24/7, most 6 years who don't live with it really don't understand and care even less.
As a paretn I would just want a normal announcement that X has a new brother/sister and how lovely for him. Then talk to th eparetns about what script they have discussed with their DS to say to people who are more interested in that.
Most 6 year old don't distinguish between a new born and a 6 months ol and a one year old but if the child is miuch older it may need a different message. If you are keen to help him I would suggest you place your energy in discussing with the parents the most appropriate way to support him going forward (if necessary).
Personally I would be horrified at a public announcement and using my family as an education exercise (I know you don't mean it this way but its the default position of many educators and I have had to explain to them that DS's start in life is not for public consumption unless he chooses it to be)