A. The fact that it has been going on for so long, happens only occasionally, and he otherwise seems well, are all very good signs that this isn't something more serious or that your doctor is missing something.
Although I'm sure that it seems like a lot to you, once a week or even 2-3 times a week, isn't really 'all the time.' Most people would consider 'all the time' several times a day.
Still, if you are not comfortable with what your doctor is saying, you might ask for a second opinion from a Pediatric Gastroenterologist. Or perhaps ask that some further testing be done, like with an Upper GI.
It may also help to keep a formal symptom diary, where you record when he vomits, what he was doing (like eating or crying), and what he had last had to eat and drink.
Keep in mind that this is a common occurrence in younger children. Some toddlers vomit when having a tantrum or just crying, others when they have too much food in their mouth, and some for no reason at all.
Since his episodes are so infrequent, it likely isn't simple reflux. But he may indeed have a sensitive gag reflex. A food allergy or intolerance might be another cause, especially if you can link it to a specific thing he is eating.
Another condition that might cause similar symptoms is delayed gastric emptying. Children with delayed gastric emptying have slower gastric emptying times than other children. That means that the things that they eat and drink stay in their stomach longer and explains why they may vomit the previous night's dinner the next morning. It is sometimes treated with the medication Reglan, although many parents report side effects when taking it, or the antibiotic erythromycin, which increases gastric emptying time.
Again, since you aren't comfortable with your doctor's diagnosis, you likely just need to get a second opinion. Warning signs that would make a second opinion even more important include if he isn't gaining weight well or is losing weight, is often fussy, isn't developing normally, or if his symptoms are occurring more often.