How to get my Yorkie to use his back legs while swimming?

When he swims, he only paddles with his front legs and doesn't kick with his back legs. Because of this, when he tries to swim, he slowly starts sinking. I tried to see if he could figure it out by himself that he needs to kick his back legs as well, but no luck. So I got a doggy life vest, and he wears that now. But I would like to see him to be able to swim without it. Is there anything I could do to help him? Do I have to physically kick his back legs for him?? Even with the life vest he doesn't kick his back legs. Any ideas?