A bit of info from Robert Sherlock's service records:
Served (no dates given)
1 year in the Penisula
4 months in Gibraltar
11 months in North America
3 years and 4 months with the Army of (Recupation?) in France
7 years and 5 months in the Mauritius
Most of what I can see is regarding an event in 1833 (in Mauritius), which cost him his arm.
He stated he was out walking with two of his children. They were returning home and had to go up a steep bank. He went first, but knowing the children would need a hand, lowered the butt of his rifle down for the first one to grab on to in order to pull him up. Somehow the weapon discharged shattering his upper arm. Long story short, it was amputated later that night.
The investigation (mandatory to prove accident vs anything else) into this included testimony from one of his sons present, a James Sherlock. James stated he and his brother were following their father home; unfortunately he never said his brother's name.
In February 1834, he wrote that he was no longer capable of carrying out his duties and asked to be discharged, adding that he wanted to remain in the colony. The deputy Adjutant General of Port Louis wrote on his behalf that Robert "was an old and deserving soldier who might yet have served some years but for the unlucky accident which lately caused him the loss of his left arm and whose unexceptionable character gives him the strongest claims to any indulgence."
I don't know if the son James will help or hinder the hunt, but good luck! It doesn't appear that Robert ever went by the Stewart name. Adopted perhaps?
Hopefully you can track his other children down for clues?