As a disclaimer, many people posting have much more expeirence diving that I do. But...we went through the same thing while getting my son certified with SSI 2 years ago. I call him a 'skinny dink' because he has a very...ah...streamlined body. Basically he's thin as a whip and has a six pack from playing soccer since he was 4. I'll share our challenges :
Wetsuit - Go to a LDS and try on multiple sizes and buy there even if it's a notch more expensive. The LDS is providing a sizing/consulting service that you're paying for. Make sure the suit fits and isn't too big. Because my son has no fat on his body, we also invested in a 3mm vest/hood that has helped when diving in the Gulf. We've bought two wetsuits and 3 pair of booties since starting because of growth.
Fins - We found a pair of Aqualung fins that had pockets that gave his bootie a little room. We slapped small size heel springs on and he hasn't had a bit of problem out of them. Due to growth, we will probably have to replace the springs with mediums by the end of this year.
Regs - You can get boil and bite mouthpieces that are better suited to smaller mouths. Strongly recommend his alternate air source on a bungie/rubber necklace....he'll never loose it or have to sweep for it.
Weight Belt - 'Skinny Dink'. He has no hips. Tried webbed belt with hard weights and a compartmentalized belt with soft weights. They both would slide down his hips way too easily for my liking. This dictated some kind of integrated weights for comfort as well as safety.
BC - We tried a variety of BCs. XXS rental equipment was HELLISH. It was either poorly maintained or would leave raw skin from chafing. In the end, we found a weight integrated BC that fit perfectly. I debated it for two weeks and decided *not* to buy it. Why? While I can afford a new wetsuit/booties every year as he grows....I can't afford a new BC on a regular basis. Oxycheq has a SS Travel Plate and an Ultra Light Travel plate. I ended up coupling the Ultra Light Travel Plate with an Oxycheq 18 pound wing and webbing. Attached to the webbing are two XS Scuba Weight pouches. All of this has worked flawlessly and only required minimal work to adjust the sizing a couple of times since we purchased it.
Good luck and I hope you have a great time together.