I have been teaching my two older boys French since the eldest was 1. They are now 2 and 4. I am basically fluent in French, and my original idea was to use immersion. But it was not really immersion because no one else in my family speaks French, and because English is our first language, there just was not enough French around to count as immersion. When my older son began talking, it became clear to me that just showing French DVDs, playing French music, reading stories in French, and speaking to my son occasionally in French was not teaching him as much as I'd hoped. So I changed my tactic. Now when I read a new French story, the first several times I read it I read a line in French, then translate in English. After a few times through the story, I begin reading only in French, but sometimes I stop and ask what a word means ("Qu'est-ce que ca veut dire?). If my son doesn't know, I tell him in English. I also ask him questions, in French, about the pictures. For example, I might ask what color the driver's hat is or what the kitten is doing. This helps me to gauge his comprehension and also helps him interact with the story. I found that my son's interest in French increased dramatically once I started to explain things in English.
We also have a 45 min French class once a week. I mostly use French songs and finger plays, and I design props to help my kids understand the songs. We really like songs by Alain le Lait, but we also use some songs I bought from Amazon.fr (Petits Lascars are good).
Little Pim DVDs are also worth checking out.
Until my third child was born, I was hosting French class at my house for other kids once a week. I am taking a break now but will resume once I get my schedule under control. I think I'm too far away for this to be helpful (I live in Littleton), but I'll mention it just in case.
There are some websites that have free games for learning French. My class website lists some, so I'll just provide the url: http://www.eieiobabygear.com/french_class/webresources.htm. My favorite site is Poisson Rouge.
If you do not speak French yourself, you probably can't actually teach your son to speak and understand French without the help of some French speakers. But you can definitely make it easier for him to learn a second language later. So even if progress seems slow, don't be discouraged! Any little French you and he learn will probably be helpful.
Feel free to contact me privately if you want more information.