Choice for growing teenager

Hello.
My son is 13. Five foot 9 and about 60kg. He has some intermittent aches and pains, mainly related to sport ( lots of cricket and a bit of rugby) eg thighs and knees and sometimes upper back muscle tension.
We reckon the projected height in 3-4 years will be 6 foot/ 6 foot 1" and final weight around 76kg.
We have no idea what mattress to choose for him- or even what degree of firmness - as we have never been able to experiment much.
I was on the verge of buying a SIMBA mattress but then read an article by you.
We have a small double ( four foot wide) frame.
You help is much appreciated.
Cheers
Helen
PS Regarding my son - referred to in previous question ( age 13) - the bed base will be slatted ( it is a low floating-type bed base).
The current mattress is a futon because his last bed broke when jumped on by a load of boys. It had a Harlequin Deluxe Postureline 1100 mattress on it which did not really give a really comfortable night's sleep . We are open minded about budget but could you suggest a range? THANK YOU :-)