First check the way he plays, if he's facing the monsters he's gathering (because of the parry/block/dodge factor) and check if he's poping his defensive CDs when needed. Check he's using his tank skills (Defiance/Shield Oath <if it applies>). Also, low DPS requires longer tanking times, try to buff your tank and help the DPS. Ask him to mark mobs, so DPS can easily kill monsters before the tank takes much damage or you run out of MP. Might sound elitist, but also check his gear and give him some advice on it. Being a low level dungeon, doesn't mean that your gear should be -15 levels (or more) outdated.
I play healer as well, and I had issues with ungeared tanks, but since I play tank as well, I start with basic advices based on his play style as I mentioned. Then I move to gear. Sometimes you can expect a negative answer from them. Can't help it I guess. I rarely run out of MP because I play SCH, but yes, those are my two cents.
Edit: And what Elurra said. It might be hard if you have SCH and no BLMs in your party. But if you do, help him with that and tell both the DPS and tank to not attack those targets.
