The debt is usually capped at 350 ducat. I have seen situations where Father and son/heir both has the Jews Moneylender debt. Once the father dies. It get merged into one penalty. IE it won't go up to 700 to pay off.
The penalty is not that bad. It incur a -10 opinion (don't recall vs who) while you don't pay it and haven't expel the Jews.
I am going to assume you are playing as a catholic. You have a few option to pay it off.
Also if you plan to borrow yourself to pay off the son's debt. Be sure to save about 50 ducat because it grant 300 while pay off is 350.
You could ask the pope for the money if you are in a position to do so.
You could set your ambition to have a war chest (enough money to wage war) and build up your wealth via tyranny event fairly quickly. Just be careful to not stack tyranny (-xx opinion to all subjects) since some of it transfer to your son rule. In some case your subjects may be rich enough to donate to you.
You could go steward focus and earn more money but those event may not trigger fast enough. This is more of a tax boost move.
You could shift some of your subjects levy/tax law toward the tax. Church subjects are usually more favorable toward tax law. This is more of a long-term strategy.
You could find some disloyal subjects that may be likely to plot against you. Try to detect the plot and arrest them. Force them to pay you to release themselves. You can also go to war and get some prisoner this way as well.