Solomon is the man who finds the eponymous diamond, but he is also a husband and father trying to do his best for his family in a malignant world. While the audience is primarily entertained by the hunt for said diamond, Solomon is also looking for his family who've been nabbed by the bad guys, well one set of them anyway. Wife and daughter turn out to be OK, pretty much, but son is taken and trained as a child soldier. The relationship between Solomon and his son turns out to be crucial, exactly as it should be.
The more I think about it, the more I'm inclined to see Solomon and son's story as an allegory for a father fighting for access to his son against malicious forces who are determined to keep them apart and to use the son as it suits them. I'll let you figure out who I think all the various bad guys might be. Melodramatic, perhaps, but I think the connection is there - in Solomon's anguish, determination and fear and in his son's trauma for being made to endure experiences to which no-one should be subject.