I think Soun’s shady brother put some sort of spell on him so he can’t speak the truth.
Maybe. But he can talk about it with everyone else so that feels illogical. This hasn't been the most logical story so that could be what's going on.
You know, now that you mentioned it... The brother doing a spell that somehow only applies to the demon guy does seem like it would fit in this weird plot incredibly well.
I think the spell is discussed with more detail and context in the light novel but I was not able to make complete sense of the machine translation when I read it.
Plots in plots in plots... And all of it would be resolved if the MC would just tell the truth.