The second part of the phrase refers to Nong Ker and not to a boy named Jong ; the word จอง (jong) means to book, to reserve. So I would translate like this : if interested by Nong (Ker), you can book him.
BTW it seems to me that "K" is not related to the boy but rather to a certain part of his anatomy (I wonder which one ..) which would be pretty, slender and long.