By using this spell on a party member would it increase the range his reach to 10. Enlarge. The target's size doubles in all dimensions, and its weight is multiplied by eight. This growth increases its size by one category-- from Medium to Large, for example. If there isn't enough room for the target to double its size, the creature or object attains the maximum possible size in the space available. Until the spell ends, the target also has advantage on Strength checks and Strength saving throws. The target's weapons also grow to match its new size. While these weapons are enlarged, the target's attacks with them deal 1d4 extra damage . With it saying the weapon grows as well would this increase his threat range for Opp attacks?
Under the combat rules in the PHB it states (I highlighted in red):
Most creatures have a 5-foot reach and can thus attack targets within 5 feet of them when making a melee attack. Certain creatures (typically those larger than Medium) have melee attacks with a greater reach than 5 feet, as noted in their descriptions.
As such, whilst large-sized creatures might often have reach, this isn't guaranteed, so is not a benefit gained from the increased size.