His work shirt! In the 1860's, working class men often wore a wool or linen work shirt or overshirt (like a smock) in order to protect their clothing from the items they were working with. Since Samuel is portraying a farmhand, this was one of his most necessary items. Beneath he is wearing suspenders and a white shirt, with a pair of gray wool trousers. I plan on adding a white collar and cuffs, as I took inspiration from this piece:
Now for the particulars!