I would second the suggestion of waiting awhile and not treating it right away, unless it's already at the point of being, you know, REALLY bad.
When DS was very young, maybe 3, he had his first dental appointment. They told me he had SIX cavities and that I was naive and wrong for using fluoride-free toothpaste with him and needed to get him on fluoride right away. They also wanted him back in a couple weeks to fill them.
I never made the appointment. And maybe 6 months or a year later, we went to another dentist. Why so long? It didn't seem to bother him, so I kind of forgot about it. Anyway, when we went to a new dentist, he said yup, he's got some cavities. Two cavities, and just little ones.
So I don't know if the first dentist was a quack, or if they somehow healed over that time (we weren't using xylitol or anything). And I'm sure that your own FIL isn't trying to scam you! My point is just that... there's not really any harm in waiting. There's a chance it may heal (I personally haven't read the research on that, but many folks do say it), or if it doesn't, you might feel less squeamish about the needle and freezing etc for a child who isn't QUITE as young as he is now.
Kids get cavities. It's not your fault, really. When I was a kid I had HORRIBLE dental hygiene. It's gross to admit it, but I went a few years where I might have brushed my teeth like once a week... and only because, you know, mom was watching me... I think in my late elementary and middle school years. I just hated doing it SO MUCH. I think maybe I had sensory issues or something.
Anyway, I'm sure my teeth were gross, there was always plaque and tartar to scrape off at the dentists, I'm sure I had bad breath and I had bleeding gums (not good).
But cavities? Nary a one.
And not due to a healthy diet, either. I had food issues too lol...
I didn't have a single cavity until I was like 19 years old, and I probably had the worst dental hygiene of anyone I know.
My first husband, who always took care of his teeth, has a mouth full of metal. My aforementioned son takes after his father. Despite the questionable diagnosis of that first dentist, he has had numerous cavities already (he's 10), including one that got abcessed. His dental hygiene is not pitch-perfect, but it's decent, and is certainly better than mine was at that age. And he eats a good, healthy diet.
I really believe that there is more genetics and things like mouth chemistry at play with cavities than dentists tell us.