Hi Karenmo,
The choice of which material you purchase would be based on personal preference (usually memory foam, polyfoam, or polyfoam) but the quality and durability of polyfoam depends on its density (see the foam quality guidelines here). Convoluted polyfoam can be comfortable (the convoluting makes the layer softer than it would otherwise be) but it’s also much less durable than a softer solid layer of the same quality material (see post #2 here). It’s a good “cheap” solution for those that need more comfort and pressure relief on their mattress or when durability isn’t as important a factor in your purchase.
Post #2 here and the posts it links to has some guidelines for choosing a topper that may be helpful. There is also a link there for some toppers of which I am aware that have return policies.
You mentioned that the mattress has plastic around it (institutional) which would affect how it feels and also the sleeping temperature (plastic can’t breathe to wick away moisture and carry away the heat). With no air conditioning and your son being a “warm” sleeper, memory foam would probably be something to avoid. Latex is also the most breathable and coolest of all the foam materials and memory foam is generally the warmest with polyfoam in the middle. Polyfoam and latex are a faster response material that feels like you are sleeping more “on” the mattress. For latex, a good “average” choice generally would be in the range of 2" or so.
Phoenix