G.C.
A child (esp. his age) should ALWAYS be well-supervised around water (pools, lakes, etc.). There should not be a situation where he would be able to get to the pool when no one was right there with him.
If you are afraid that while watching other child(ren), you might not get to him as quickly in order to keep the others safe, you should consider also putting him in a life-jacket when you are outside in a location that has a pool. They make them for children and it is well worth the investment. Although you should note that the life-jacket does NOT remove the need for supervision.