He woke up during the day in Ramadan and found that he was emitting maniy (semen), and he does not know whether it was a wet dream or masturbation; what is the ruling on his fast?

Firstly: 

Wet dreams are a means of releasing sexual
energy in both sexes, and the individual plays no part in that; rather they
happen to him when he is asleep, and are part of the nature of the human
being. The individual will not be brought to account for that, because the
Pen is lifted from one who is asleep (and his deeds are not recorded). This
has been discussed previously in fatwa no. 9208 

Secondly: 

If the emission of maniy from you during the
day in Ramadan happened because of a wet dream, then this does not
invalidate the fast, because this is something that is beyond a person’s
control and he cannot prevent it. Allah, may He be glorified and exalted,
said: “On no
soul does Allah place a burden greater than it can bear”
[al-Baqarah 2:286]. Ibn Qudaamah
said: If a person has a wet dream, it does not invalidate his fast, because
it was involuntary on his part. It is similar to the case if something
enters his throat whilst he is sleeping.

End quote from al-Mughni by Ibn
Qudaamah (3/128). 

The Standing Committee was asked about a man
who had a wet dream during the day in Ramadan: what is the ruling? They
replied: Whoever has a wet dream when he is fasting or is in ihram for Hajj
or ‘Umrah, there is no sin on him, he does not have to offer expiation, and
it does not affect his fast, but he has to do ghusl from janaabah if maniy
was emitted.

End quote from Fataawa al-Lajnah
ad-Daa’imah (10/274) 

Thirdly: 

If the emission of maniy during the day in
Ramadan occurred as a result of masturbation when the person was awake and
aware of what was happening, then his fast is invalidated and he has to
repent to Allah, may He be glorified, for having masturbated. 

Firstly: because it is a forbidden action,
as has been explained previously in fatwa no. 329. He
also has to repent from masturbating during the day in Ramadan, because by
doing this deed he has violated the sanctity of the fast. Then after
repenting, he has to make up that day on which he invalidated his fast.
Shaykh Ibn Baz said: Masturbation on the day of fasting invalidates the fast
if it was done deliberately and he emitted semen. He also has to make up
that day if it was an obligatory fast, and he has to repent to Allah, may He
be glorified and exalted, because masturbation is not permitted at the time
of fasting or otherwise. It is what people call “the secret habit.”

End quote from Fataawa ash-Shaykh Ibn
Baaz (267/15). 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy
on him) said: If the fasting person masturbates and ejaculates, he has
broken his fast and he has to make up the day on which he masturbated. But
he does not have to offer expiation, because expiation is only required in
the case of intercourse. He also has to repent from what he has done.

End quote from Fataawa Arklan al-Islam
(p. 478) 

As you are confused and you do not know
whether this maniy was emitted because of a wet dream or masturbation, it is
to be taken as the result of a wet dream, because the basic principle with
regard to the sleeper is that he is innocent and is not accountable at that
time. So we should act on the basis of this principle and not do otherwise
except on the basis of certainty. This ruling does not change even if the
emission of maniy came about as a result of you playing with your private
parts, so long as you were asleep, because an action done by one who is
sleeping is as if it did not happen, because he is not accountable at that
time. Shams ad-Deen al-Asfahaani said: There is no differentiation between
one who is dead, one who is sleeping, and one who is out of his mind with
regard to the ruling being waived for them, because we know on the basis of
reason that one of the conditions of accountability is that one should be
aware of what he is doing. 

And Allah knows best.