CIW, hi
Having spent my life working with children I do think your son is too young to be affected by the situation,having said that they do pick up on your mood swings and atmosphere.
I would think it more than likely he was just under the weather,over tired or teething.
Do you and your wife try to keep him to the same routine in each others care? ie regular tea time ,bath and bed or whatever pattern his night time follows. That he does need children need routine it gives them a great sense of security.

Ah the heart and the head are often at loggerheads I guess that makes us human.
You sound like you are doing pretty good all things considered.
Did yu decide on your night out yet?