Times of loss like this are tragic and life changing. I am sorry for your loss and the loss his family is experiencing. In times like these I chose to honor those lost by living life the same way why we did when they were here.
I can't tell you what to do but, if the 2 of you rode quads and enjoyed it then I think honoring him by continuing to ride would be a great thing to do!
He is on that great ride and you can bet he is in the throttle having a great time!! If he was here he would probably be telling you. Ride on my friend ride on!