Man, 6 days later and not a single lurker here has answered your question? I find that kinda interesting in itself. 

Perhaps like myself, no one actually knows. However, I have used him over the past several years and I'll try to give you what little insight that I know of. When Son kills someone with his A2 ability and his splash damage spreads, I have seen a hit do 50k on one champ and 20k to another on the same team. However, I'm not sure if it was due to affinity OR perhaps the 2 champs Def difference. If I remember right, the 20k champ was Elsa (weak affinity) and the other 50k was Alice (also weak affinity). But, was the damage less because of Elsa's defense OR was I able to land a crit on Alice and not Elsa because of the weak affinity?

I wish I could help you out better but I'm hoping that p[erhaps bumping this back to the top for you might make someone put their eyes on it and have a better answer for you.

Good luck bud.

1) if by affinity you mean eg force vs magic, then, no, it won't affect that, as that isn't a passive skill or mastery.

2) the surplus damage is treated as attack, so, it will also be reduced/increased by affinity.