It's certainly possible, in lower level hockey to have a d-man who can't skate backwards and still do alright. When it's just hockey for fun (I understand, you always want to win and do well, but you are playing for fun and the love of the sport) and it's at a lower level, which I'm assuming it is since he's a new player, it's not such a big deal that he can't skate backwards right now, but it's certainly something he's going to want to address whether he intends to progress to a higher level or not, so I would suggest working on it with him in the warm-ups and asking if he has a pair of rollerblades he could practice with off-ice a little bit, as another poster suggested.
Not that you gave any indication that he'd be sensitive to you suggesting he needs to learn to skate backwards, but if you feel he'd be a little insulted or abrasive, just compliment him on his work ethic and tell him he'd be even better if he polished that skill. It's nice to have a good d partner. I played defense in college, and even though it's a totally different ball game at that level, one thing that I think was underrated by some of my teammates and buddies (and maybe is underrated in hockey in general) is the chemistry and communication that a defensive pairing can develop. If you like the way he plays and can compliment him on his game and give him constructive feedback that will help him improve, you guys can enjoy playing together, make both of each other's lives easier and have a great time on the ice by communicating and forming that d-man bond. I swear, I was only able to be half the player I was in college (which was only 1/4 the player a lot of the guys I was playing with was, but still) because my partner and I became really good buddies on and off the ice and learned one another's strengths and weaknesses inside and out. It sounds like you really appreciate his tenacity and enthusiasm, so maybe you guys can develop that kind of on ice bond. Unless you were just praising his tenacity because you didn't want to be a dick about saying he couldn't skate lol.