To bare the mechanics of the
self-inflicted insult suffered by our brother,
as uncovered in
TheGreatEscaper’s solution :
Number Slope™ puzzle
hint assumes that
S, O, I and L are equivalent to 1, 2, 3 and 4,
in a consistent order observed within each
set of linear squares within a tetromino.
The circled cells determine that
O is 1 or 4 while S and L are in the middle.
Thus I is also 1 or 4.
This is the direct result of that partial ordering.
The circled cell group determines
the order as O S L I or its reverse.
The 4×4 grids proceed to specify all but
two inconsequentially ambiguous areas.
Letters at dotted corners
can now be copied to the 5×5 grid.
Row 3 -T.
Giant upside-down T  tetromino
sighting — as
the dotted corners are brought together.
A new mystery letter ▯ presumably is equivalent to 5 in the order.
The circled subsequence L S ▯
establishes the complete ordering as...
... I L S O ▯.
The 5×5’s bottom row’s direction is unknown initially
but S is in the middle either way.
This forces L into the center cell
and the rest follows readily enough.
At the top of the 5×5 grid we at last see our
brother’s embarrassing confession.
“ I L O S T ”
Subtle reinforcing hints.
Why is the mystery letter T
when E could just as well spell an equivalent message?
there are 5
known to science,
T being the one not in play here,
simply have not yet discovered an E-shaped tetromino
As subliminal reinforcement,
a gigantic upside-down T  tetromino
is formed within the 8×8 combined grid of row 3 -T above.
How were numbers and letters matched up?
The 4 tetrominoes in play
enumerate the 4 known grid letters,
by counting slopes.
This order is also found, in reverse, among the circled letters
in row 3 above.
But the puzzle’s poser has mentioned
that yet one more reinforcing detail
hides in plain sight among the grids:
The letters in the dotted corners of each 4×4 grid are I, L, S, and O in that order. (This means that the grids are ordered by the number of slopes of their corresponding letters.) But the 5×5 grid seems to be "paired" with the second grid: the space between them is shortened!
Just as the 5×5 grid is an "exception", the letter T is an exception to the rule about number of slopes: it has two slopes, like the L it is paired with, but it is the fifth letter in the sequence. Also, it must be moved to the end of the solving order, just as its letter must be moved to the end of the sequence order.