Depending on what time of day you are at the pool, it may be hard to find a chair without a towel on it already to relax on in the first place. But, if I remember correctly, there may be some spots over by the shallow, sandy pool area where you may be able to see where he would cross to get to the slide stairs. As far as him getting tired, there are a ridiculous amount of life guards so if you lost sight of him I'm sure he would be fine. But I always feel the need to hover and never let my daughter out of my sight so I'm a fine one to talk
But honestly I don't think I ever sat down on a lounge chair at the pool but still found it very relaxing ( I think at Disney any time you are not inside a park is pretty peaceful). How much time are you planning on spending at the pool? Enough to sway you resort choice on that alone? Are you the only adult? If not, maybe take turns watching the boys.