If it's been that long, and he isn't willing to pay to reimburse the cost you spent on the gun to repair it (that it sounds like he agreed to) or to pay for shipping it back, then sell it and recoup your costs.
I've done it before for someone who didn't pick up their gun for 6+ months (and pay for the repair as well). I gave them a deadline of 1 or 2 months in the future, and informed them if they didn't pick it up, I would do with it as I chose. They never picked it up. I sold it.
Might sound harsh, but it's what you have to do sometimes.
Airsoft Sales and Repair/Upgrade Services