What time is he fed at night? You may try splitting that into two meals, one closer to bed time. Dogs, unlike us, produce bile before eating. It may jyst be an empty stomach issue.
What food do you feed and have you changed foods?
Sometimes dogs will produce large amounts of acid if they go too long in between meals. While this is more common in bulldogs, it can happen in any breed. It sounds like this is what is happening with your lab. You can apply a quick fix to this by feeding him a biscuit or two very late at night. This should keep his hunger down and the acid in his stomach to a minimum until breakfast. Even though adult dogs are traditionally fed only once a day there are circumstances (like yours) where it is more prudent to feed a few small meals several times a day. Of course, this will alter his bathroom habits as he will have to be taken or let out more often than you are used to, but if feeding a couple of smaller meals along with a late night snack prevents the vomiting, it's worth it. Please keep us posted and let us know if this works.
Ghilly