I have absolutely no idea what could have caused him to stop repairing after Vault 15. All i can remember is that i went through most of the NKR's remaining quest's, then drove to Reno for selling some stuff, came back to the NKR and told him to wait there because i wanted to bring the cyber dog out of town. After that he rejoined, i drove to Vault 13, did everything i could do there and moved on to the military base. There it began... he refused to repair. Another strange thing about him was, that he didn't wanted to equip anything else than a 10mm pistol, a 44. handgun, an ak112 or ak198 assault rifle. Everything else was refused by him, even if it was the only small arms gun in his inventory and he was set to ranged only. Btw, changing his armor won't change anything either.