I think it is likely this is just an arbitrary pattern
The main reason is that the SDH says "[knocking rhythmically]", as opposed to "[knocking in morse code]".
However, if we treating the pattern as morse code, we have several options. The pattern is 2-1-3. But the final knock for any section is relatively ambiguous. The possibilities (with their translations) are:
.. . ... (IES)
.- . ... (AES)
.- - ... (ATS)
.. - ... (ITS)
---- ignore for now, this line will be important later ----
.. . ..- (IEU)
.- . ..- (AEU)
.- - ..- (ATU)
.. - ..- (ITU)
(disclaimer: I've never seen this show, so I don't know if any of those letters have significance.)
There are two ways to see if he is trying to do a dit (dot) or dah (dash). The first is timing, and the second is visual.
In morse code, a dit has a length of 1, a dah has a length 3, the space between letters has a length of 1, and the space between words has a length of 3. Acronyms count as a single word, so the only then we need to pay attention to is how long the space is between is letter.
After watching the clip you put in the question, I do not believe there is enough time between letters to allow for a dash. At best the spaces have a length of 2, which is still 2 short of allowing a dash to finish and the space to occur. The observant will notice that it is also too long to count as a dit followed by a space, but let's set that to the side for now.
The other way of guessing the dit/dah nature of the knock is to watch his hand. Does it rest for the possible dahs? Or does it do a quick dit and then wait. Unfortunately this data contradicts what the timing tells us and looks like he is rest his hand on the door after the first and second section of the pattern.
So if we allow for a poor sense of rhythm, and the instinctive nature to rest our hand on the solid surface while "typing" a dah, the best we can say is that he is trying to knock one of the options above the line you ignored earlier.
However, since the various methods contradict, and the second time he knocks was different (in timing) from the first, and the subtitles only say that he is "[knocking rhythmically]", I feel it safe to assume that it was an arbitrary pattern that he picked to be uncommon in nature, thus allowing a reasonable certainty that it was him should Eleven hear the knock.