I would think a metal container would be most suitable- how about a large biscuit type tin or a new petrol container that you get from service stations? That type of thing (not necessarily a petrol container iykwim!)
Also you could add a DVD recording (or CD) of a special message. Another thing I thought of would be the front page of the newpaper from his 1st b'day or the whole paper if you can fit it? Also, a 1 or 2 dollar coin minted in 2009? A copy of his favourite book or toy (if suitable- don't put the original in!!)??
Such a lovely idea. Also, why don't you get a book (like they have at weddings) where everyone writes a lovely little message inside and get b'day guests or family/friends to write a message in for when he is 18?