I never heard of this rabbi, and don’t know who backs him. In general, even in Israel this is rare. Personally, I am not at all in favor of young marriages.
However. . . there are a very few Chassidic communities in Israel where the accepted thing is for girls to get married at 15/16. Everyone in these small communities for the most part does it, and it seems that the girls really do “grow up” at an earlier age. Perhaps because in their minds they always viewed that age as an adult. I am personally familiar with one couple from such a community — when they got engaged the girl was 15 1/2, the boy was just 19. (By the time they were married she was just 16!) They have been very happily married for over 10 years with a number of kids, and the community as a whole seems to do fine.
Keep in mind, they are living almost a “shtetl” lifestyle. Generally speaking all the parents, siblings, etc. live in the same small neighborhood, helping each other as necessary. People live very simply and do not require a lot of money, and the men often end up learning part time and doing small odd jobs or teaching on the side, and that covers their expenses. Housing is dirt cheap, many people have their own gardens and even lifestock, etc. You cannot compare these communities to those in the US or even the rest of Israel, and certainly to condemn their way of life without any knowledge of it seems unfair.
Keep in mind that even in the US their are certain communities (such as some of the Sephardic communities) where girls routinely marry at 17. While I can’t imagine that for my own daughter, if it works for them, who am I to decide that it’s “too young”.