I've been testing around with him and I must say, he's quite the rock solid support. I wouldn't suggest playing him like a champion that can be picked in any situation...
cough 
cough
More over a niche pick and situational, but he's nice when you have a 
mid/top and a 
laning with you. Double knock-up OP.
Any suggestions on how I can further improve as a support with him? I haven't been able to do much with the 2.5 million current Sona mains. :/
pretty much any tanky person played as support is OP. There are people who rank to plat just doing duo tank bot.
Personally, I love playing Mundo Support. This is the post-game of my most recent game. The invisible cleavers are a nice touch when they happen lolol
as far as sona goes, I just constantly cleaver to force her to use her mana up by my 6, then just dive and wreck house. Just like in the attacked image, I started off with a DK when Vi flanked with low health, because sona was oom and vi didn't check :o Ez was low at beginning, so had to run
Otherwise (or if she rushes/steals blue), you just have to play cautiously until you can kill her quickly, before she can get off enough heals, then go for ADC. Ali does this well, as does Blitz and Taric. Because of knockups/stun.
Also, always make sure you get her to blow flash early. Otherwise she can leave you high and dry for a jungle flank with your failed gangbang :o I usually just turn on my fire and run at her from bush and she panic-flashes.
