New discoveries have found exceptions to Mendel's basic laws, but none of Mendel's things have been proven to be flat-out wrong. Hail to the Father of Genetics ! The name is ma-ma-ma-ma-ma-Mendel.
Dental crowns should last on average from 10 to 20 years. Crowns are still subject to fracture and cavities, so it is important to take extra care in brushing and flossing around crowned teeth to prevent them from needing replacement too often.
Children get a total of 20 teeth, they will fall out as the adult teeth are ready to erupt. The root of the baby teeth will disolve as the adult tooth comes into position. The pattern of tooth loss is not exactly the same for each person, but usually starts with front teeth and ends with the baby molars, and takes years, around age 5 to age 12. If you haven't already gone to a dentist with him, find him a good pedodontist for a check-up. A pedodontist is a dentist who specializes in kids. Good luck.