Unless he refuses meat when given it because he only want vegetables (for whatever reason) he's not a vegetarian
I think this is it. It's not just not eating meat for a week or two 'accidentally' but stating a requirement when asked, requesting a vegetarian option if it's not available from a selection put in front of you and having nothing, or just the salad, chips, bread etc if it's not available.
Because otherwise, it's likely that you're just choosing the food that you want that happens to be vegetarian. It's not making a concious choice to be vegetarian, it's seeing something you like and thinking 'I'll have that'. That's what I'm like. I'm not vegetarian and would never describe myself as such because I do eat meat occasionally (maybe once or twice a month) and I eat fish regularly - a couple of times a week at least.
I'd also never use any of the wanky modern terms like part time vegetarian or flexitarian because I happened to go a few days not eating meat. That's being an omnivore, it's how people are supposed to eat.
But I think this stems from meat eating being seen as the default for a lot of people. In a restaurant, they automatically choose the meat option and don't even look at the vegetarian meals as they seem to put them in a special weirdy category for the vegetarians only that you'd only eat if you didn't eat meat. Not true.
But some of my favourite foods are in that category. In my city there's a few Indian Street food places that I could happily eat at daily because I love just about everything on their menus. Of course, most of what they sell is vegan and it's all vegetarian. But that's secondary.
If I have pizza, I always have mediterranean veg, because that's what I like and don't like meat on pizza, to me it just doesn't go. What gives me the rage is the type of pizza place that will sell 20 different pizzas and half of them will have some sort of processed meat on, 6 will be chicken and then about 4 that have none of these on and at least two of these will be margarheta or 4 cheese, leaving one or two with any kind of topping other than cheese that's not meat. Ridiculous.
Falafels, hummus, pittas, salads is another of my go tos that I have at least twice a week.
And then there's all the normal food that even non vegetarians will regulary eat without thinking much about it - cheese sandwiches, most soups, cereal, cake, crisps etc. If you have these for breakfast and lunch, you're not 'being vegetarian' you're just eating food.
But I also eat fish and chips and that's almost always cooked in beef dripping here, so even the chips aren't vegetarian. Also roasts or BBQ meat occasionally.