The most important factor, IMO, is your budget. Normally, I would say stay at the YC as its proximity to two parks and great pool does mean that it would work really well for you and your son. Also, the rooms are large and comfortable with balconies which is nice. Finally, you have stayed here and loved it!
However, I have never found a vacation where I was worried about the price to work out well for me personally. So, I would really look at the numbers (in CDN$$) to determine a budget that would be comfortable for this time around. All of the resorts that you have chosen are enjoyable - we have stayed in all of them and enjoyed our stays. We deliberately stay at all levels of resorts. One reason is differing financial considerations and different travelling companions, and the other is that we simply enjoy all WDW resorts in different ways and like to experience the nuances of staying in them all. This is your opportunity to try something new! It is exciting to find a new resort to love!
CBR currently has all the construction finished except the Skyliner. So, it is currently on my short list for a revisit. The pool here is great for young children, IMO. At 9 I believe that your son would enjoy the pool here. There are also other 'quiet' pools scattered throughout the resort. Be aware that this resort is large and spread out so there can be quite a distance from your room to the main public areas (themed pool, dining etc). The new dining here sounds to be good as well. I am looking forward to trying this one out again soon.
CSR is simply awesome as a moderate. It feels a little like a blend of deluxe and moderate (which Disney seems to be blurring the lines between with the new tower construction). We love the additional amenities and the pool which is second only to SAB. We also enjoy the ambience of the lake setting. I think that both you and your son would enjoy the pool area here. The slide and large hot tub are great.
This is also a large resort, but doesn't feel quite as large or have quite as confusing a layout as that at CBR.
I am concerned that they are changing this resort with the Gran Destino tower so that the quaint charm will be lost, but it should still tick all your boxes. The CSR is nice and centrally located so transportation, although by bus, has always been positive in our experience. NB: It is a convention resort, so that can mean groups of people in suits around the resort. This is off-putting to some people.
POP is a value resort, albeit my favourite. It has the newly renovated rooms that you may enjoy. However, I would stay in a moderate, if the budget allows. There is no slide in the pool here, which you indicate is important to you. Nevertheless, if this is the most comfortable price point, then I would stay here happily. You will also be a short walk to AoA for their over the top theming and food court. Some rooms here have really nice lake views. The rooms are the smallest of those that you are considering BUT with only 2 people will easily meet your needs.
FWIW: my opinion
from what I know of your situation, I would probably choose one of the two moderates - either CSR or CBR. Take a look online at the pools and various pictures of the resorts to decide which appeals to you and your son the most. If budget permits consider a preferred room location, but don't worry about getting this. We have stayed in preferred and regular rooms. While preferred locations are more convenient, the difference is only a few minutes to the main areas and buses have stops all around the resort so that it is never far to a bus stop.
Good luck choosing!
