Hello, and contratulations to your nephew!
Becoming an Eagle Scout takes months and years of effort and involvement in Scouting. The final requirement is the individual scouts 'Eagle Project', which is usually a considerable community service project.
This is not just a scout function for any scouts at a certain level.
This is a real personal achievement for the boy.
It is not like a Scout Troop Commencement. But an event for the new Eagle Scout, planned by him and his family.
My son's Eagle Scout Ceremony involved catered food (not fancy, a nice barbeque) and a short slide-show of him (and some of his troop-mates) as they enjoyed scouting activities and achievements... Starting with a photo of my son as a kid in his first, brand new, cub scout uniform... And ending with his Eagle Scout portrait type photo. (this was set to music of 'The Climb', sung by British X-Factor winner Joe McElderry)
In my son's scout troop, the ScoutMaster likes to make sure that there is an impromptu 'roast', where those who know the Scout well, tell funny personal stories, like "That time at camp when he.... "
It was a very, very, special evening.
Do attend, if at all possible.
Gifts are not the focus.
But, a gift might be nice.
Seriously, with teens, some cash or a nice gift-card in a Congratulations Card would be great.