Because you are using now in the main clause, the present tense is also appropriate. In this case, however, I would use could—simply because it's what happened in the past that has enabled him to do something.
The sentence, as written, is something of a garden path variety. The mixed tenses seem strange at first, and don't really make sense. Things only become clear once the sentence is read in its entirety.
It would flow better if you reversed the elements, changed the use of parenthetical information, and moved now to the start of the main clause:
Because, earlier that day, he had saved the life of a fourteen-year-old boy who remarkably resembled his late sibling, he could now picture his brother's face more clearly.
This now gives it the simple form of because X, Y. Since the because part is in the past, the mixed tenses are fine.