For clarification nika explains to nami that he can hard if he thinks about summer. But he doesn't want to "fake" a response. That he wants to get better. So it is psychological and I don't think it has been made clear whether mom is aware of impotence. Nika has not been in relationship since summer I believe so it may not have arisen (pun intended).
Nika and Noji talked about it when they got back from Book Club, he told her that his depression was affecting his sex life, she confirmed that she didn't know for sure but had long suspected it.
Nika "I don't think you know how much my depression affects me."
Noji "You need to be a little more specific Nika... You don't really talk to me... I can only assume, and if I assume the wrong thing... It's going to be weird."
Nika "About sex."
Noji "Oh."
Noji "Oh I... Umm... I figured as much."
Noji "You never left any traces... I suspected it."
Noji "Not like- others in the house..."
And somehow, thanks to this dialogue, and some previews of Noji and Nika in the pool from the second season, it starts to seem to me that Nami is not the only one who will soon be concerned about treating the wormy.