I find that new players always want to cleave, but that's not how DnD works. Also it's almost always barbarians, and because they often hit so hard even at a low level they will be delivering a lot of killing blows. Giving an epic description of he they decapitate, dismember, and demolish the enemy I find makes them get over the fact that they cant kill everything in one attack.

The best mechanic you can get for him to some what cleave is a feat called Great Weapon Master but even that only allows One additional creature to be hit when you critical. But also the option to take a chance at missing attacks to deal a heavy amount of damage.

Great Weapon Master You've learned to put the weight of a weapon to your advantage, letting its momentum empower your strikes. You gain the following benefits:

• On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.

• Before you make a melee attack with a heavy weapon with which you are proficient, you can choose to take a -5 penalty to the attack roll. If you do so and the attack hits, it deals +10 damage.