How is the inheritance divided among the mentioned heirs, considering that one of the paternal uncles has been missing for 20 years?

If a woman leaves no heirs other than her husband and a son, the husband receives one-fourth due to the existence of a lineal descendant, as Allah Almighty says: {But if they have a child, then for you is one fourth of what they leave, after any bequest they [may have] made or debt.} (An-Nisa: 12). The remainder goes to her son by way of ta'sib (residuary inheritance), based on the Prophet's (peace be upon him) saying: "Give the prescribed shares to those entitled to them; then whatever remains is for the closest male heir." There is nothing for the other mentioned heirs because they are completely excluded by the son (exclusion of deprivation). Thus, the inheritance is divided into four shares: one share for the husband and three shares for the son.