He would be helping to run the depot. The soldiers (who were usually waiting for a discharge hearing) are called 'inmates' on the census. The Invalid Depot was like the Barracks Department, ie staffed by discharged NCOs. To find his regimental career you will have to wait for Findmypast to put earlier records online. They have only released papers for men discharged 1883 to 1900.
I have found a marriage in Meerut, India on 7th January 1852, between John Sutton (aged 28) and Susan Handlin (aged 15). If you can find it in the regimental BMDs on Findmypast the index should tell you his regiment.
My guess is that his father-in-law was a soldier as well
Ken