We do our main praying in the morning when we drive to school. I will ask the kids for a volunteer to be the "prayer leader" who will go first, and then we go around the car and everybody else has a turn to pray. Often when we get to Caroline (our middle daughter) she will refuse to participate. I then usually tell her "it's Ok-- you don't have to pray out loud-- God doesn't mind if you pray in your head", and we pass her and go on to someone else. I am hoping that even though she is not praying like we'd like her to she is still learning a good daily prayer example , and is encouraged to communicate with God in ways that are comfortable to her, and she realizes that NOT praying is not an option. And usually by the next day, she will be out of her funk and want to pray out loud again.
Maybe you could encourage smaller prayers/praises during the day that seem more "fun" (draw a picture of something you're thankful for, learn a new song about Jesus, buy some beads and string together his own rosary, etc.) -- rather than a lengthy on your knees session at the end of the day??
Maybe at nighttime, you and DH can do the praying for/with him and not force him to unless he wants to. Maybe once the pressure is off, he will want to spontaneously participate like mom & dad do.
Let us know if anything works-- I am always looking for new tips to help encourage prayer in my kids! (and me too, for that matter!