Mattress for growing teenager. R90 Endo, sports therapy etc

Hi,
We have a John Ryan originals mattress with your previous 50% latex topper which is brilliant, if a very tall combination!
Now looking for a mattress for a 12 year old boy. Clearly within the life of this mattress, he will grow a lot and go from probably around 50kg to around 90kg. He Loves sport and I've seen some "sport" mattresses with testimonials from sports teams etc, such as the R90 Endo from sportsleepcoach and the sports therapy mattresses. They seem to be memory foam/cool foams over a reflex base.
In shops, he loves the Tempur cloud and the Latexsense Opulence (talalay over Dunlop) and generally likes a soft to medium feel.. He also gets very hot in bed, hence tending away from traditional memory foam.
Any ideas about what in your range would suit, without breaking the bank (!) and what do you think of the "sports" type ranges?
I've also looked at the eve/Bruno/sleeping duck/leesa type mattresses. They seems to be latex/memory foam over a firmer base. Is that firmer base necessary for support?
And one final questions, I also need a new mattress at some point. Can the VAT be reclaimed on any of your mattresses for a chronic disabling back problem? The Mammoth, healthbeds memory flex etc can all allow a VAT reclaim.
Sorry for so many questions!
Thanks