Does the father have to go before a judge upon his release for the father to modify the temporary custody though he had full custody previously?

Yes. The temporary Order would be the prevailing Order until a new Order is entered.

Unless the temporary order says, upon the release from jail, custody reverts to the father, he must file a RFO to modify custody.

It depends on what the temporary order says; if it says nothing about custody once father is released then you need a new order.

Your facts are not clear.? You say temporary custody was granted. A family member (her family or yours) could only get such through a third party custody proceeding as they had not standing (not part of the case between you and the mother.)? The answer to the question is, yes, you have to present yourself in front of a judge in the third party case to say I am now out of jail, I have solved the deficiency I had for having lost custody for a time, and I should now have custody of my child(ren.)? If your original order giving you custody was from a dependency, then the question is would the dependency court agree you have resolved any problems with you having custody.? The third party court might say, prove you are okay now - meaning get a psych evaluation or pass a drug test given over two or three weeks, just to give them some basis for proving your statement.? Keep in mind it is the safety of the children, not your possessary "right" to have them. So expect to have to prove you will keep them safe, no matter why you got arrested.? Now if the mother with the help of family got the court in the file where both of you had standing, the answer would be the same, yes, you have to appear before a judge to undo what was done because of your arrest, but the process will be more complicated.

Whenever you want a modification of a an agreement in Family Court, you need to go back before the court.

If he wants to make sure he doesn't' get into trouble, yes. He should consult with a local attorney.