C.S.
One thing that stuck out to me: don't put the burden on your guests. Having guests write letters to your future 21-year-old would be a task most wouldn't be up to.
Same thing for buying gifts for an older child. It's much more difficult for the guest to do that.
Why don't you do a photo slideshow with some great music for his first year of life? My sister made one for my daughter and I still love it. Burn it to a DVD and you've got a great memory!
The tablecloth idea is nice--and if you don't want all the extra "toys and clothes he doesn't need" tell your guests that no gifts are needed at his party. :) Some will still bring some, but some will welcome the chance to save some money. With fifty adults coming, you hardly need that much stuff. :)