Well, he did do a series of punches for Boros.
Well maybe he didnt actually punch boros, and use the wind of the punch that he blows like when genos asked for duel at the mountain area. He stopped his punch next to genos's head but the mountain got splitted into two pieces (didnt harm genos tho since its probably not right in front of his head and his punch is not actually aimed at genos because he knows his punch will harm genos) if he did really punch boros he should punch him really hard like when he punch those monster (like aimed and punched through exactly at their body) He couldve stopped his punch in front of boros in each punch to use the wind of his punch to harm him since probably he want more challenge. ╮(￣▽￣)╭ because in the end he die in one punch, saitama probably just want to encourage him since he was actually a good guy or something (could potentially a good ally for the heroes)
Those were not his serious punches but only the last one could have been considered a serious punch.
Exactly ╮(￣▽￣)╭ hes technically still one punchman
If saitama blows more than one punch then hes technically not onepunch man anymore. That cocky little shit will die soon.