Why would 9 str hold him back if he is using finesse weapons and does not have proficiency in martial anyway?
Biggest thing holding him back from a melee build I feel is the lack AC (mirror image/mage armor helps this) and the fact that eldritch blast is so good.
In melee you have a hard time maintaining concentration for Hex as well, losing out on bonus damage, so your have to resort to Helish Rebuke or Armor of Agathys, witch provide less consistent damage over the course of a fight and require you to be hit, which you don't want to be hit in the first place due to lower health pool (made up for a bit with temporary life buffers).
Seems like the "idea" is to create a highly agile, hard to hit melee character with Darkness and Devil's Sight (bugged) and Mirror Images.
If melee your options are:
Rapier (1d8+dex bonus) so about 4-11 dmg.
or
Dual Wield Daggers (1d4+dex bonus *2) so about 8-14 dmg (I believe offhand weapons are not supposed to apply dex bonus unless 2hand fighting is selected as a skill, but they currently do)
Remember Eldritch Blast counts as a ranged attack, so being in melee puts you at a disadvantage and reduces chance to hit.
I'm not saying the strength by itself holds him back, I'm saying that both his dexterity and strength are garbage for using weapons so he can't use either of them. Low strength is fine if the dexterity is high.. his dexterity is not high though. He only gets a +1 modifier which is horrible
