I'd say you're definitely fine to start without him as long as they know the plan -- as Pame said, make it clear. That way he can grab food on the way and just join you for dessert.
Another option if you have a bit of time after 8pm: You could offer to order something for him so it would be ready right at 8 when he gets there, if you're comfortable with that. If the menu is available online have him pick his top 3 dishes (top 3 because whenever I look at the menu in advance, the dishes I get my heart set on are all sold out) and you can write them down and let the waitress know when you get there. Will make things faster all around.