You stated that they are replacing ill fitting crowns. As long as the temp crowns are well fitting and cemented properly there should be no problems with getting decay or with them popping off. If one does pop off you can always recement it.
Esthetic crown lengthening can make your canines larger if you have altered passive eruption. Meaning your teeth have not completely grown through the bone. If your teeth have grown completely through the bone then a simple gingivectomy can be done to make the teeth longer.
If you are looking at long term solutions, either option will work. If you are looking for long term aesthetic solutions, I would go with the implants. This will give your son 1.) more teeth to function with, 2.) a more natural smile, and 3.) more confidence in that his teeth and...
