They look healthy but very small. 9 weeks veg from seed under a 600 watt they should be much, much bigger by now.
You can see from the pics that he's running a CFL at the moment. Maybe he's swapped in the HPS by now.
Personally I would stick with soil until you've had a few grows. Coco can be liability and massive learning curve if you over feed even slightly. Soil can be a lot more forgiving.
At the moment he's not measuring pH/EC.. Slowly does it I'd say.
Plants are looking fine by the way, good work