I think it's great that you want to ge your son into swimming lessons, but about the only thing you can do for him right now is sign up for mom & tot classes. I am a swim instructor, and most children under 3 don't have the coordination to swim independently in the water. More importantly, they don't have the sense to understand the dangers of water. I have seen, time and time again, overly proud parents because their 2-year old can do a "doggy paddle" or retrieve toys off of stairs or even in the shallow end. At two, there is no sense of fear. What often happens is they don't understand that they need to come up for air, and take in water and come up choking and terrified. If this happens, it can take years to get your child comfortable in the water again. I have seen this happen and I have spoken to many parents who have experienced this.
Bottom line is go to the mom & tot classes to keep your child comfortable in the water. If you go to a pool party or public pool/beach where there are alot of kids and you can't spend the whole time in the water with him, keep a vest on him at all times if you are afraid he will get away from you. And don't take it on and off of him when you are at the pool, because he may forget that he doesn't have it on and jump in. However, if you do this it is VERY important that on other days you spend one-on-one time swimming with him without his vest so that he doesn't become dependent on it.
Park districts and many high schools have swim programs that are very affordable. Some are better than others so you'll have to ask around. But the HS's all keep their water temperatures fairly cool for their swim & water polo teams. The Y, if you have one close has excellent programs as well. You can also check local health clubs, such as Lifetime Fitness and Palo Health & Fitness.
Ideally, you'll want ratios of 4-5 students or less per instructor. You also should watch a lesson before signing up. That way you can see how involved the director is--if he/she is watching the instructors or not. Often the instructors are HS or college students and if the director is not around, they can tend to chit chat more with other instructors and just go through the motions with their students.