You misunderstood. I did not say finishing the OW cert course in the ocean is necessary. When I recommended doing it in the ocean I hadn't yet understood that you don't intend to dive in the ocean but rather plan to dive inland sites like quarries. Most people prefer ocean diving, so that was my assumption. A bit of advice that is commonly given by members is to take dive training in the kind of place you intend to dive. I wouldn't call quarries "lame" by any means. The few I have dived have been cold and less than great visibility--in other words, realistic diving. If quarries are your thing, then it makes sense to do the OW part of the course in a quarry.Yeah... but a lame quarry is still not the ocean dive that you think is necessary.
What I would like to convey, though, is that Blue Grotto is more like a fishbowl--almost artificial in how diver-friendly the environment is--than all but the lamest quarry and certainly nothing like the ocean. It's simply my opinion that a student (or your son in this case) won't get as much out of doing the dives at Blue Grotto as he would in the ocean OR a quarry.