The Boer War medal records are all on Ancestry under C Stevenson 3710 in UK Military Campaign and Award Rolls 1793-1949, there are three records.
His Great War medal card is also on Ancestry under Charles Stevenson 3701 in British Army WW1 Medal Roll Index Cards 1914-1920. This shows he went to France on 17 September 1914 so, as ShaunJ said, indicates that he had probably finished his earlier service but still had a reserve commitment and was recalled at the outset. There are medal roll entries also under UK WW1 Service Medal and Award Rolls.
The pension record is on the Ancestry bit they call Fold3 under 3701 Charles Stevenson in UK WW1 Pension Ledgers and index cards, there are 2 records. Fold3 requires an additional subscription although there is a free trial option that you can cancel when done.
If you don't have a sub to Ancestry, your local public library (if not axed already!) may have access on their computers.
MaxD
Specific links follow.
