If his energy level and demeanor are normal and his eating, water and elimination habits remain stable, I don't think you have anything to worry about. Maybe add some vegetables to his food…peas and carrots? He might need some nutrient he's not getting in his regular food. Kind of like me...there are times when I can't get enough salad and other times I don't miss it.
Dogs eat grass for a variety of reasons, not all of which is to vomit. I have two that like to graze on a certain type of grass as though it were a delicacy. This grass does not irriate their GI tract and they never vomit but they love to chew on it when they find it (which is hard in Arizona).
I also have one dog in particular (though the others also experience on the rare occasion) that gets an upset stomach - especially in the morning after an overly long fast (in otherwords after I have slept in and his meals are spaced longer than 12 hours apart).
He will go out and chew a very fibrous and almost prickly long bufflegrass that definitely irritates the GI tract and makes him vomit. His upset stomach is most likely due to the bile acids that accumulate in his stomach in preparation for food. When the food is not forthcoming and his stomach is static - the acid just sits there like sludge and makes the dog nauseous. The older he gets - the more this can occur due to the more static his stomach becomes as he ages.
There are a couple of things that help 1) do not let your dog have an overly long fast period. I try and give large cookies at bed time and this usually helps 2) talk to your vet about motility drugs such as metoclopramide (Reglan) that helps the stomach to regularly empty itself whether food is present or not - this can keep bile acids from accumulating and making the dog feel bad - you can just use at night if the morning seems to be the time he feels nauseous 3) if you have an alternatve vet who uses Standard Process Whole Foods Nutrional products - ask them about the use of Choline. This product worked wonders on my guy in addition to not letting him have an overly long fast - which is hard to control sometimes.
If this continues, never consider it "normal" - it is not normal to feel nauseous repeatedly every couple of days - therefore it is never wrong to get a full blood panel work up - esp. now that he is 8 and considered geriatric. It is always good to have base line values in which to compare as he gets older but you might be able to catch something early.
When Tyler (12 yrs) starts to eat grass I cook up a batch of greens (broccoli) for him and feed it to him as treats. This seems to take his urge for vegetation away immediately.
I also see a change in his eating habits now that he's a bit older. Some days he doesn't want to eat when it's 'dinner time'. This can lead to empty tummy syndrome and the occasional bile urp. If I give him a slice of american cheese first thing in the morning or a peanut buttered cracker, this keeps the upring to a minimum and seems to stimulate his appetite. Same for in the afternoon and evening. An empty tummy can soon turn into an upset tummy.