Of course YABU. I'm very glad my parents never pressurized me re grades, simply encouraged me to try my best. I did well at school, and my God did I pity the poor kids at GCSE who had to do well or else they'd get in trouble. He's a child op, a child. He's got the next 60 plus years to work hard, let him be a kid. He's only 7! The Scandinavian countries have one of the best record of children doing well in school, and they don't even start school until 7!
You get children nowadays in tears because of SATs, worried they need to do well etc and with this kind of attitude it's obvious why. In year 6, I wasn't even aware SATs were happening - just took the booklets we had to do as another bit of work, certainly didn't get stressed over it, and I'm sure that's down to my parents attitude of 'be good, be polite, try your best because trying is the most important thing'.
Ok, so he had an off day. Have you never had a day at work where the figures won't sink in your brain or customers are particularly irritating? Have you never had a day where you got a pizza on the way home rather than home cooking your tea? Have you never ever had a bad day or a bad week? We're none of us perfect OP, and having off days/weeks is alright.
'Revision' 'Chill time' He is a child. Bar the homework schools set he shouldn't have enforced revision! Yes he needs to go over the spellings, but make it fun. Don't 'cut down his chill time'. So because he did badly he doesn't get to watch his fave tv programme for eg and rather has to go over the spellings, that's a way to make him resent education and learning from the start. There's a reason the foundation phase (or stage, in England) is based on learning through play, because that's the best way to do it.
By all means effort should be encouraged, but then that comes more into play later on, when he's in secondary, when older, not when so young. Just tell him well done for trying and that that was what was important, that he gave it his best shot.