Keep offering him different foods. Try different ways of fixing it. Like mashed, cubed, diced, chunked or whatever. A pellet diet would be a good idea. Try warm foods too. Cornbread, wheat toast are good too. Do not give up your bird well be a healthier bird for it. It can take at long time so just keep offering it.
Beak grinding is a good thing. As far are how to rehab an older grey you'll have to let someone else give you advice on that I have never done that.