How to make kids speak our native language??

Hello All....Hope all of you are enjoying the week and looking forward to the weekend.
Summer has started in Austin and we will reach 100 degrees soon. We try to take the kids for swimming at least once a week to the pool in our community. When my son was around 9 months we got him a pool toy like the one below, which he used to enjoy a lot. It had two holes for the kids to stick their legs and swim. If you have young kids buy one of these and see they will really enjoy in the pool while you can swim in peace just holding the string with one hand.

Coming to today's topic, one of the biggest challenge that I face now , is to make my son speak in our language (ie my mother tongue). I think everyone one of you who have immigrated to a different country from yours will face this issue. I am trying everything I can to make my son respond in my mother tongue "Tamil" , these are the things I do:

*Always talk to him at home in Tamil and will respond to him in Tamil.

*Give him special incentives when he talks in Tamil to us and to his sister

*I try to tell him that grandparents dont know English and you have to talk in Tamil 

*Though when I Skype with my parents they will ask "How are you doing?" in English to him ,so I keep telling them to talk in Tamil to my kids

*I got some Tamil alphabet books that I read to him now and then

*Our friend suggested to play some Tamil Rhymes on YouTube. So I play this series "Chellame Chellam" every now and then.

In spite of all this I feel like I am slowly loosing the battle. Please let me know if any of you have been able to successfully make your kids talk in your native language and what all you did to achieve it. 

It is so funny when I was young I would be punished if I conversed in "Tamil" in my school. I had to converse in English. Now I am facing exactly the opposite situation with my kids.Your feed back and comments are always welcome :)