How long will he more than likely get for a sex with a minor charge if he is currently on felony probation for assault?

He could be looking at a long sentence. First, if he is on probation he could get time for that without even being proven guilty, only a preponderance of evidence is required for a probation violation. Second, the fact that he is more than 3 years older means that it could be in the area of 10 years give or take.

Statutory Rape is covered under California Penal Code Section 261.5 (d). This states: Any person 21 years of age or older who engages in an act of unlawful sexual intercourse with a minor who is under 16 years of age is guilty of either a misdemeanor or a felony, and shall be punished by imprisonment in a county jail not exceeding one year, or by imprisonment pursuant to subdivision (h) of Section 1170 for two, three, or four years. Child molestation is covered under California Penal Code Section 647.6 (a) (1). This states: Every person who annoys or molests any child under 18 years of age shall be punished by a fine not exceeding five thousand dollars ($5,000), by imprisonment in a county jail not exceeding one year, or by both the fine and imprisonment. Both offenses are registerable offense under Penal Code section 290, if he is convicted of either or both of the offenses the court will require that he register as a sex offender for the rest of his life.

I don't know without knowing more facts of the case and his entire criminal history which will determine his sentencing guidelines. However, I can tell you with almost complete accuracy that it will be a prison sentence and probably at least several years. Of course, that also depends on what he is ultimately convicted of. There may be problems with the case that a good attorney can use to get some of the charges reduced or dismissed. Also, if he is convicted his probation would be revoked on the assault charge and he could be re-sentenced up to the maximum amount on that offense as well. He is in a lot of trouble and facing serious time in prison as well as being placed on the sex offender registry. He needs to be represented by an experienced criminal defense attorney that specializes in sex crimes.

It will depend on all of the circumstances but it could be decades.

He can be revoked on existing probation. He can face up to the maximum in his jurisdiction on each of the new charges. All charges can be ordered to be served consecutively. In short, he needs counsel as soon as possible.

If the age difference is 15 and 21 in CA he has committed a felony. That combined with the probation violation means he needs a lawyer as soon as possible if he doesn't want to become a permanent resident of the state prison system.

That depends on a number of factors , but if he is a predicate felon and she was 14 it seems likely he would get a few years in prison for raping a child, maybe a lot more. He should retain a good lawyer and plead the case out so as to get a break if the evidence is overwhelming or go to trial if there is little evidence and no confession.