Is it permissible for a son indebted to his father to take from Zakat funds to repay his debt, given his ability to repay it within 5 or 6 years from his own work?

If your debt to your father is immediate and not deferred, and you are unable to repay it now, then you are considered among those in debt (gharimin) and are eligible to receive zakat (definition) to repay the debt. However, if your debt is deferred, there is a difference of opinion among scholars: the soundest view is that one should not be given zakat for it, as there is no current need for it. If your father gave you the money and did not specify a term, the original principle for a loan is that it is immediate, and you are permitted to take from zakat if you are unable to repay it.