Can an aristocrat disinherit his heir in favour of a younger son if the estate is entailed?

Nope.

There are different types of entailments, especially now, but the type of entailments we typically see in the Georgian era are fee tail males, entailing the estate and all estate-connected assets by way of primogeniture. Primogeniture means firstborn.

Primogeniture ensures only the firstborn son in the family inherits. There is no will, only the primogeniture law. Entails and primogeniture are two different things, but they’re often used together since aristocratic entails, along with many landed gentry entails, were based on primogeniture.

The wording for primogeniture-based entails is “the male heirs of his body.” How primogeniture works is only the legitimate (sorry, illegitimate and/or adopted sons) firstborn son can inherit. This son will inherit everything. Anything entailed by this method cannot be divided amongst the children, as only the firstborn son inherits.

The inheritance follows that direct line of the first son, then the first son’s first son, then the first son’s first son’s first son, and so forth. The only way a younger son could inherit is if the first son and all his male children were deceased or he beget only daughters or no children.

Not all entailments were set up using primogeniture, but those cases were almost exclusively landed gentry, not aristocracy, since the aristocrats did not arrange the entailment details. The entailments tied to aristocratic titles and whatever assets (estates, money, etc) accompany the title were set by the Crown, which favoured primogeniture.

With primogeniture, the firstborn son in the line is “heir apparent.” His inheritance is indefeasible. His eldest son will also become heir apparent, and onwards down the line of firstborn sons. We often see the phrase “heir presumptive.” The heir presumptive is not a firstborn son, and his inheritance is conditional. There is only an heir presumptive while the title holder remains childless of sons. Often, the heir presumptive is a brother or cousin, and his eligibility to inherit can be usurped by the birth of an heir apparent, ie a firstborn son of the title holder. Consider the heir presumptive a kind of temporary heir.

Read more here:

https://www.paullettgolden.com/post/entailments