Here in the original manuscript, Oliver Cowdery initially skipped the first instance of the phrase my son; but virtually immediately he supralinearly inserted it (there is no change in the level of ink flow). Later in this verse, we get a second use of my son (“yea and again I say unto you my son”). The strong parallelism in this verse argues for the use of my son in both cases.
Either reading, with or without my son, is theoretically possible. In his discourses to his three sons (Alma 36–42), Alma uses the phrase “I say unto you” 14 times. In four cases, that phrase is followed by my son (twice in speaking to Helaman, here in Alma 36:21, plus once in speaking to Shiblon, in Alma 38:3, and once in speaking to Corianton, in Alma 39:6). But in the ten other cases, my son does not follow “I say unto you”. In each case, we therefore follow the reading of the earliest extant source, thus “I say unto you my son” twice in Alma 36:21.
Summary: Maintain in Alma 36:21 the two occurrences of “I say unto you my son”, the corrected reading in 𝓞 for the first occurrence and the invariant reading for the second one.