I wouldn't worry about the time of day that he is eating. I would worry about the quality of food he's eating. Any food you don't consider of meal quality I would quit buying or replace with a more nutritious alternative so that if he's getting the bulk of his calories from snacks, they are just as nutritious as his meal would have been. Since it sounds like their may be an attention/control issue here, I'd consider setting up some snacks in a place he can access and letting him do so as he wishes to, with the caveat that he must eat at the table.
As for dinner, I would serve dinner. It is what it is. I would not offer an alternative. I would not let him have other snacks after dinner. I wouldn't worry about if he doesn't eat. It's quite common for kids to eat more of their food in the first part of the day. Other than growth spurt times, I feel like my kids basically eat what I think of as two meals worth of food a day--they each have one meal they tend to eat well, one meal they eat half of, one meal they pick at, and then a snack or two. I don't worry about it.
Unless you had a major failure to gain weight issue, I'd avoid losing sleep about it. Since the pestering for snacks is annoying, I'd try to create a way for him to have more control there and that way it's easier for your not to worry about it.