1. Faded Trench Coat, You can only got this after finishing "Unlikely Valentine" Quest. After saving Nick and bring him to his office at diamond city Ellie Perkins (Nick Secretary) will ask you to become Nick partner like watson and giving you Nick outfit.

2. Ballistic weaves Modification, You can only get this mods by finishing "Jackpot" missions given by P.A.M from The Railroad. After finishing the mission go directly to Tinker Tom and he will teach you how to add ballistic weave to outfit.

3. Armorer perk Level 4, for better upgrade.

1. Bring "Faded Trench Coat" to armor workbench, and apply Ballistic Weave Mark V.
2. Do trade with nick and give the Armored Faded Trench Coat to nick, and you will see nick able to equip it.
3. And here you go, Nick outfit already got upgraded. He now more durable to taking damage from bullet and melee weapon. You now can give him melee weapon to make him rush and knock the enemy.

As bonus, upgrading him somehow will fix his hand. make him have better looks now.