I tried a latex mattress when we bought our current bed and founf it was too soft - well I thought it was but in saying that the one we have ended up with hurts my back

Not sure about weight. He will be tall though and the bed (and whatever mattress is on it) will end up being replaced probably sometime towards the end of primary school. At the moment his weight is a full curve lower than his height.

ETA: Jas thanks for that, I might just go to a few places and ask.