WElcome to TE! if his finishing time on this event is really important to him, maybe you could find someone else to ride with?
There's a ride that my husband and i did last year... he stayed with me for the ride.. next year he wants to do it alone to see how fast he can finish. And I can really appreciate that.
He shouldn't ALWAYS have to ride at my speed, but then
sometimes, if he wants me to ride with him, he MUST ride my speed, because I'm never going to be as fast as he is..

And then there are tandem bikes