He was fasting and he told someone that he was not fasting; what is the ruling on his fast?

Praise be to Allah

If when you said “I am not fasting”, you intended to inform him that your
intention was no longer to fast, and that you had now broken your fast,
then you did break your fast, whether you ate something or not. 

In
the answer to question no. 95766, we explained that
if a person forms the intention to break the fast whilst he is fasting, and
has firmly resolved to do that, without any hesitation, then his fast
becomes invalid, and he has to make up that day. 

But if when you said “I am not fasting”, you were simply lying and you did
not intend to stop fasting, then this comes under the heading of a lie, but
you did not break your fast because of merely saying it. 

The one who breaks the fast is the one who breaks it by eating, drinking,
having intercourse or otherwise, or who forms the intention to break the
fast and has decided and firmly resolved to do that. But if someone says
that and is lying, he has not broken the fast. 

You have to repent to Allah for having told a lie, because lying is a
blameworthy characteristic that detracts from the reward of the fast.

Please see also the answer to question no. 37989 

And Allah knows best.