He's meh. He's outclassed by other tank supports in lane, only adding a small amount of poke and his trade off for this is his ult.
Don't get me wrong, the ult is great and can work well in a teamfight, but so can Leona's, Alistar's, Thresh's, Blitz, Braum's... And they don't give up lane presence for it. The only reason I would personally pick him is if I knew I was going in with a Yasuo and my ADC knew that we were going for a farmy, "Let's chill at the tower" lane.
The few times I've come across a Malph support, he got a couple of hits in early with his Q, but after that and until 6, he was like a pinata for me, procing blue gold item and bandit whenever he showed up, very easy to bully. At 6, just had to let them push and keep poking the ADC, Malph didn't (and most likely won't) have the gold to do much damage, and outside of a slow and a knock up it isn't really too hard to deal with him. Hell, just save exhaust and the fight can turn around much easier than if the above listed tanks were there.
The reason Naut works well (in my experience) is that he acts similar to Leona and Thresh. He chains CC and is a disruptive monster, both of those points work without gold investment. Malphite doesn't have enough disruption in his kit to make it worth it to me, and again should only be picked if you know your team and you set up a certain comp, and even then you may be better off.
Short version: Malph is a 5/10, can work, wouldn't invest time into learning him as a support.