There are so many ways to honor a loved one at weddings. Some have placed a rose in the seat that would normally be taken by your loved one. Memorial candles are big, however might be an issue with an outdoor wedding as you stated. If you are having wedding programs, remember your loved one with a nice poem or verse. Have a poem read at the wedding in his honor. Place a small table with his picture on it either near the guest book, or another visible place. A picture and a single long stem rose can look elegant. Maybe you have a picture of your FH and his father you might like to use.
I'm sure you will find a beautiful way to honor your lost loved one...Good luck!