I've been developing a problem that I don't think they're taking seriously enough. I don't want to insist, it'd seem like I was telling him how to do his job.
You need to discuss what you think the problem is and ask how they can fix it.
If they explain that there is no problem then you should expect that waiting for them to fix it is pointless.
If they agree that there is a problem then you need to discuss a schedule by which time you should expect this matter to be dealt with.
The reasoning behind that advice is that you have brought this up with them and they have provided you with an answer. If you didn't understand part of the answer, why it's not a problem, then that's the portion that needs a better explanation.
If you know you are correct and that they are wrong you need to take your business elsewhere. Indeed, with something more complicated than what you are qualified to know about, you can seek another opinion.
The question arises: If the next dentist agrees with the first will you accept that, or if two out of three isn't enough will it need to be three out of five? You need to obtain a satisfactory explanation that you understand otherwise you will continue to question what is being said to you.
If it's necessary to book an hour long appointment to discuss this then that's what you need to do. The dentist probably won't mind being paid to talk instead of working on your teeth, since you are paying for it. If you try to book for a procedure at a fixed price then want a free consultation thrown in for good measure that comes out of the dentist's pocket instead of yours.
You should be able to ask for a recommendation to an expert in the particular field of dentistry that is specific to your problem. There are a limited number of experts whom have taken the extra training to specialize and being recommended to them is necessary to book an appointment in most cases. You could visit another dentist and ask if they will refer you to an expert.
Only you can decide if the problem lies with: the explanation, the listener, the dentist. If you are in any pain you should change dentists. If you are worried that one tooth is a little crooked then perhaps the dentist doesn't believe that you would want to pay over a thousand dollars to fix it.
You need to explain what you think the problem is.
You need to obtain an explanation that you understand.
You can ask if they will remedy your complaint even if it's a trivial matter.
To spell out the reasoning, for others whom would say that the reasons were not provided, it's your mouth and you are paying for it. Your dentist should take your complaints seriously and you should take an expert's advice seriously. If you don't have confidence in your dentist you need to go elsewhere.
I'm not scheduled to see him again for a few months, so if I go with the "tell him" option do I need to call him first or just make an appointment with someone else myself, and what do I say once I go back next time?
Call your dentist soon and book an appointment for a consultation. Use the time to discuss this beginning to end to your complete satisfaction. You can ask for referral to an expert if you feel it's necessary. If they refuse then you need to decide between staying or seeking another practitioner.
If you consider yourself friends with this family then you should feel comfortable discussing your care with them, they might wonder about whether or not you accept their advice if you go behind their backs and ignore the advice you have been given.