I've been living over seas for 12 of the past 17 years and have had FPO & APO addresses the whole time, and I've never had a problem getting anything mailed to me.
Try going down to your local post office. As long as you have it packaged safely (so it won't break) and it's under 70lbs (i think that's the limit), you should have no problem sending it.
Just make sure you send it insured/registered AND MAKE SURE YOU KEEP THE RECEIPT!!!! I recently bought a bayonet from a guy on arfcom and when the box arrived, it had a big hole in the bottom and the knife was gone. I CANNOT file a claim till the sender sends me the original receipt for the shipping insurance.
You will also have to fill out a customs form (small green card that has your address, his address, and a description of the contents). I think there is also another form you have to fill out, that also gets stuck on the front of the box.
If it's too heavy, you may have to go through a freight company...
ETA: If all else fails, ask the guy for his command / unit's local mailing address (street address) there in japan. Every military command / unit stateside & overseas has a local address (street address) for the shipment of large items. UPS and FEDEX can and do ship to these addresses. If he doesn't know, let me know what his unit / command is and I may be able to help you out.
I'm stationed on Guam right now, and we ship stuff to japan via fedex all the time. If he can't figure it out, ask him for his "DSN" phone number and I'll call him up and help him out.
What you think you heard is not what you thought I said.