When one is saying Keriat Shema Al Hamita and just saying the 1st paragraph (as he has already said shema after nacht), does one add Kel Melech Ne’eman, or is this only if saying all three paragraphs?
It is only when saying all three paragraphs.
It is worth mentioning that the Mishna Berura writes that a person should try to say all three paragraphs of K’rias Shema when saying K’rias Shema al Hamita.
All the best
The Rema (O:C 61:3) writes that when a person says K’rias Shema in the morning and evening (Shachris and Maariv), they should add the words the three words Kel Melech Ne’eman, in order to bring the total numbers of words in K’rias Shema to 248 which corresponds to the 248 limbs that a person has.
The Mishna Berura in Hilchos K’rias Shema al Hamita (239:1) writes that even if a person already said K’rias Shema (e.g. during Maariv), they should still say all three paragraphs of K’rias Shema when they say K’rias Shema al Hamita. The 248 words in K’rias Shema [245 words in K’rias Shema plus the 3 words Kel Melech Ne’eman] protect a person’s 248 limbs.
We may infer from this that if a person has a reason not to say all 3 paragraphs when saying K’rias Shema al Hamita, there is also no reason for them to say Kel Melech Ne’eman, since they are anyways not going to be saying 248 words.