Regular dentist visits are often more a form of preventative rather than curative medicine. Even if you take care of your teeth, brush twice a day and floss, there are some problems which can occur, and which can become more serious if not nipped in the bud.
For example, I went for a checkup yesterday, and I brush at least twice a day and floss fairly regularly. After I had my braces removed, I had a permanent appliance installed behind my lower front teeth (basically just a thin wire cemented to my teeth to keep them in place – as I understand, most orthodontic patients who’ve had braces in the last 20 years or so have them), which makes it impossible to floss between those teeth. I had a pretty severe buildup of tartar, which has caused my gums in that area to begin to recede, and now I have exposed root surface there. Not a huge problem, or life-threatening, but it’s something that won’t repair itself and that will likely only get worse as I get older.
Tartar buildup happens even to the most diligent of brushers and flossers, and it really is best to have a dentist or hygienist professionally clean your teeth to remove it before it starts to cause problems.
Even if your boyfriend doesn’t have any pain, it’s possible that he does have cavities which, as others have mentioned, can only get worse with time, and which could lead to severe decay, root canal, or loss of the tooth. Some people seem to think that just because they’ve never had any problems in the past, that means that they don’t need to go to the dentist, but that’s simply not true.
Yeah, it’s not fun or free, but like @Fiddle_Playing_Creole_Bastard said, it’s better to go and deal with the unpleasantness than to have more serious problems crop up later.