Welcome back, Isabel!

I noticed just a few things in your post that stood out to me, and will comment on those.

<< He is now 11... he is beginning high school next year >>
Is he grade accelerated? I ask because in the USA, the typical relationship between a child's age and grade level is a 5 year difference:
. . Age of student - 5 years = grade level.
. . In this case, age 11 - 5 yrs = grade 6 (USA middle school).
. . HS nxt yr, age 12 - 3 yrs = grade 9 (USA high school freshmen).
Am I understanding your child's age and grade level correctly?

<< He is quite disorganized, his notebook is full of doodles and he seems to always find shortcuts to technically do what he is required to do with minimal effort.>>
Sounds creative.
Possibly Twice-Exceptional (2e)?
Possibly a Visual Thinker, and compensating?
The book Different Minds by Temple Grandin (Nov 2023) may be of interest.

<< ... mostly happy, well-adjusted, and popular among his classmates ... He is quite goofy and likes to joke in class but he is mostly well behaved. >>
Those two descriptions seem to be compatible.
That is, if I understand him to be gregarious, social, outgoing, well-mannered, having a sound sense of empathy and support for others and a well-developed sense of self-control and self-discipline. The role of "entertainer" MAY indicate a gifted child's efforts to fit in and be accepted, and/or boredom, or and/or a strategy of 2e compensation.

Reading about Executive Function may provide insight regarding both organizational skills, and self-discipline. Here's one link, from Understood.org - https://www.understood.org/en/articles/what-is-executive-function

<< I am now trying to figure out where we stand, also because the person testing him this time won't be focused on giftedness. >>
What will the tester be focusing on?

<< He does have occasional glimpses of brilliance (for example, last year we were checking the sugar amount in a box of cereals in the store, which was 9 gr per 100 gr and on our way home he commented 1/11 of the cereals was sugar).>>
Mathematical prowess and/or an affinity for math is great, however it is not the only sign of brilliance.
In this vignette, I see a combination of curiosity and math, playing well together. smile

<< His score at age 5 (WPPSI) was:
FSIQ 131, 98th percentile
GAI 149, 99,9th percentile
>>
IQ scores are typically considered stable at about 8 years old and beyond.