My FH and I plan on having a table at the reception, reserved for all of our loved ones that have passed away. I'm going to get a 5 by 7 stand up frame and inside the frame will say something like "This table is reserved for our loved ones who are no longer with us. Though they are not here with us physically, they are with us in spirit....and forever in our hearts. Please take a moment to remember those who have passed." The frame will be set up in the middle of the table with white votives all around it. And then I'm going to get single white roses, one for every person we know that has passed, and have a tag on each stem with the person's name. The roses will be places all around the table.
Even though you're having an outdoor wedding, you can still have a memory candle. Just get a tall clear glass candle vase ~ that way the wind won't blow the flame out. Maybe you could get a calage frame or make one using a poster board, and have a bunch of pictures with fond memories of his dad. Have the frame set up on an easel with a small table next to it and the candle placed there ~ could also have a small vase of flowers on the table as well.