I would perhaps seek a second opinion. My two-and-a-half year-old daughter has a cavity on one of her front teeth. It is TINY, and the dentist indicated that it is very shallow. He will just buff it down a bit an add a sealant to protect the tooth. Even if her needs to add a filler, he said it will be white to keep her tooth looking good until she loses it in a few years. Perhaps your son's cavities are deeper or larger, but crowns seem a but much.
As for the general anesthesia part, that too seems excessive. My daughter will also have to undergo an extraction of a tooth next month, as there is one right beneath it causing her pain. (Strange to have such an early second tooth, but I did see it on the x-ray.) Anyway, she will take a sedative in the dentist office that will take about one hour to take effect. She will just then be "out of it" but not knocked out supposedly. This is ONLY because of the need for extraction. If it were just the aforementioned cavity repair, our dentist told me he would opt for novocaine and some nitrous oxide (laughing gas) only; he said that is how he would have treated his own children at that age.
As a side note, my daughter has been under general anesthesia four times, all before she turned two. As you stated, it is a big deal, and the little ones often experience about a week's worth of night terrors beginning about 5-7 days post-op as the anesthesia leaves their systems.
Best wishes in making your decision.
Jennifer