scrap book idea is good but you MUST remember that he will have to hump all that stuff back with him, when space and weight is at a PREMIUM.
So think birthday, think silly....................I would hit the local dollar store and get silly hats, paper cups, plates napkins enough for him a good few of his mates to enjoy...........and fill the box with some food goodies [don't go over board just enough so he can be silly for a few moments on HIS day..........I also would put in a balloon [helium one already blown up] that is at the top of the box when he opens it.............think of it like a mini party in a box...............maybe have a few of the stupid stiff from the dollar store in the toys section that they can mess about with and then just toss away with NO worries, so he gets to have fun on his day, he does not have to worry about how the heck he is going to get it back home.
When I did these type of boxes for my son I would write in the letter inside the box that it did NOT matter about keeping it, just have fun and be silly................but that I would like a few pictures.
I did a birthday in a box, did a superbowl in a box, halloween in a box, etc etc ALWAYS with enough for at least 8 people to share..............told him to share, enjoy, take a picture or two for us and either save it for the next lot....or dump it with out worrying that I was going to be upset or offended............the ONLY thing I ever asked him to hold on to was a Christmas hat that lit up and played songs [cost me a fortune] and he does still have the hat.............
edit oh I do like NWIP's ideas, would just be concerned about the whole weight of him bringing it BACK, but that one sounds great.......