If one should perform Ghusl (ritual bath) but he doesn’t, can he fast?

In fact, Salah is the obligation which can never be missed under any pretext or reason. Unlike it, Hajj can be missed if man is ill or poor, Zakat, can be unpaid by the poor, Siyaam can be missed during travels and in case of sickness, and Shahada (testifying that there is no Deity worthy of worship Allah Alone, and Muhammad is His Messenger) is uttered once in man's lifespan.  
Salah is the obligation that is performed frequently, and it cannot be missed no matter what, because during it man connects with Allah the Almighty. Furthermore, we observe Siyaam, perform Hajj and pay Zakat in order to keep connected with Allah the Almighty. Hence, if the purpose of the act of worship is missed, you will lose the main thing in your worship  
My advice is to go on with your Siyaam, and beware no one including me can advise you not to observe Siyaam, lest it is unacceptable. This is between you and Allah the Almighty. Well, observing Siyaam without performing Salah is pointless. Yet, if man masturbates at night, and he wakes up after Fajr time, there is nothing wrong with his Siyaam, so what should he do in such a case?  He should perform Ghusl. Although he commits a sin by missing al-Fajr, he will be in a big trouble if he misses the other prayers due to not performing Ghusl.