Hyrum Smith, the scribe here in 𝓟, initially wrote the plural Sons, then immediately corrected it to the singular Son by erasing the plural s. The use of his near the beginning of this verse and of him later on in the verse argues that the singular is correct (although one could, I suppose, argue that the him is an error for them). Earlier in this chapter, the text explicitly refers to Aaron as the son that the people wanted to succeed king Mosiah (see verses 1–3). And later in this chapter (see verses 8–9), Mosiah once more refers to this son and what could happen if he were made king. The use of the singular son in Mosiah 29:7 is undoubtedly correct.
Summary: Maintain in Mosiah 29:7 the singular son, the immediately corrected reading in 𝓟; the singular refers to Aaron and is supported by usage elsewhere in this chapter.