If he got on at Southampton then he would have boarded there. I believe you actually answered your own question, unless I missunderstood. If you're looking for a time range, it would have been sometime before noon on the morning of April 10, 1912.
Ooops, I misread the title of the thread, I thought it said "where". I'm keeping my original post intact, but thought I'd add anyway. The passengers would have been able to board throughout the day at any time, depending on when they arrived to the ship. The majority of them would have boarded when the train reached Titanic (wasn't that about 11:30?).
I think he meant which location on the ship. As per the Fr. Browne photo, it appears that there was a gangway extended to C deck, where the 2nd class passengers would have boarded into the 2nd class enclosed promenade and continued to the staircase to go to their cabins below either by the fore or aft staircase. A perfect location. Either the lift for the lower decks, or a quick descent to D deck. If you boarded early enough you could tour the 1st class accommodation.