Fwiw my DS was more of a 'glass half empty' personality early on. Anything unfamiliar, he considered very carefully whereas his younger DSis jumped right in. He thought things through, foresaw problems, worked out solutions. Spontaneity wasn't really his thing! Like you I found myself thinking why this negativity, why can't he relax.
In our case he was the pfb, maybe he picked up on some of my novice parenting worries, and by the time we had DC2 we were more confident parents.
I know a pair of brothers, the older one felt displaced when his younger DB arrived and he feels insecure anxious and the need to be ultra competitive. Not helped by the younger sibling being sunny natured and naturally gregarious, everyone warms to him. Reassurance seems to be key.
Now in this instance your DS is loving, cuddly, popular. So he saves his grumps for you? Or is like that with his dad too?
he holds his head complaining about a headache, honestly you'd think he was having a seizure. As soon as I hold his hand, his headache miraculously disappears.
Healing hands! Or more likely that extra bit of attention. As his teacher says he's fine at school I wouldn't worry. At home he can be himself and not make an effort like at school. In our house tiredness usually accounted for beyond normal strops.
By age 9 I found my DCs had worked out for themselves what they wanted to do after school; DD liked organised activities during the week, DS preferred a more free form approach. They each flourished in their own way but to this day, function differently.