Cherry Juice. Get the good natural stuff that's not full of processed sugar and drink a glass each day. At our Chiropractic clinic a lot of senior patients have gotten really good results with reduced arthritis pain in their joints from this.
Then there's also the geek project to solve the problem... build a small robot system together that he can use for soldering. Something that can tolerate slightly trembling hands while still offering solid and precise control. I imagine a simple robot arm that can attach a soldering arm, controlled by a joystick system with varying motor speed for more precise movements. Add in a large magnifying glass and an adaptable vice to hold the board and you have a precise soldering workshop.