Hi Leo
The army did not have customs officers, so he was an army pensioner having served a minimum of 12 years in the army (after the age of 18), who later joined HM Customs. Most men were discharged between the ages of 30 to 35, depending upon when they enlisted. The pension for a 12-year service Private would not be enough to live on - probably about 4d a day - so he had to find other work.
If his army papers survive they will be in Kew, in series WO97, in the section for men discharged 1883 to 1900 (or maybe 1900 to 1913). They are in alphabetical order, so you do not need to know his regiment. If you cannot get to Kew you will have to wait for Findmypast to put pre WW1 papers online ‘by 2011’.
If you cannot find his papers you may find him either in the muster books or on a medal roll. But both are filed by regiment. The last muster books end 1888 to 1898, depending upon the regiment or corps.
I am sure there will be a HM Customs archive somewhere.
Ken