I like
Malachi. I don't think
Malachi Mason would work well though; I'm not a fan of alliteration. Although it is sort of fun to say... and it's sort of alright because
Malachi has 3 syllables and
Mason has 2... idk.
I like
Kai as a full name. As a nn for
Malachi I'd probably spell it
Chi or maybe
Cai.