He's going to want to put things in it.
When you help him to decide what to keep in the boxes which fit on the bookcase, you will discover that he has beat you to it and just dumped things in.
So you make him empty them and sort it all out.
As he is sorting it all out, you discover that there are other boxes and bags filled with randomness,
So you make him sort out those as well.
As he is sorting, you decide to straighten up his dresser.
When you straighten up his dresser you discover 24 unmatched socks. So you take them out to find their partners. You also discover a garbage bag full of outgrown clothes.
You keep interrupting his sorting to hold up shirts to him.
After you finish the dresser, you decide that he could use an actual shelf to keep his books.
So you take some of your college age son's books off the shelf and put them away for safe keeping. You also discover piles of little things wedged in between books on the book case. You add these to the ever growing pile to be sorted.
Moving the books will make your realize how dusty the room is. You get the dust cloth and start dusting. As you dust, you realize that floor behind the trundle bed is awash in more little things. So you move the bed and add them to the pile to be sorted.
Eventually the pile is all sorted and manner of random items are returned to their rightful home. Your son vacuums his floor and admires his amazingly clean and sorted room.
When your daughters see their brother's amazingly clean and sorted room, they are going to want one, too.
And since your husband is only now starting on loft bed number two, you make some tea and start the whole process again.