That's it for now, give my love to your father.
Write back soon.
Someone has said that it is a mistake and one should write 'That's all for now...' Is it okay to finish an informal letter this way?
You can finish an informal letter any way you please.
If you are fussy about the layout, you might start a new line for
Give my love to your father
and possibly add something to Yours
and be sure that you wish to convey your love only to a single (living) parent (your father) rather than (both) your parents or your mother and father.
In short, there are no rules concerning ways to finish informal letters.
"That's it for now" is synonymous with "That's all I have [to say, to report] for now". Perfectly legit.