If you are able to be within arms reach and he is the only child you are supervising, then I don't think you need any kind of flotation for a swimming pool (boating, open water, etc. are different situations). We use the Sterns Puddle Jumper lifevests or a full-suit life vest when we are taking more kids that can't swim than there are adults or on a lake/boating situation. The other time they are helpfull is if you have a very independent toddler who will not stay in arms or may dart off.
Most preschool age swimming lessons do not use any kind of wearable/attached floatation, IME. In our red cross lessons, they only used the lifevests on the day they talked about using life vests as part of water safety. The rest of the time they used pool noodles or kickboards only as needed for practicing certain skills.
Mama to three boys ('03, '05, '07)