There are some complex mechanisms at work. Some of your son's may improve with increased weightbearing. You say it's worse when his foot is dangling. The foot has a "plantar pump". The veins of the foot depend on pressure and foot muscle action to pump blood back up to the calf which then pumps it further up. Veins are kind of wimpy about this on their own. There is also the autonomic issue, worse when cold. This function can get impaired in lower extremity surgeries even without CRPS and usually improves over time per my surgeon.

My foot looks normal when I am on the bike or walking (unless it's 88 degrees like the other day then goes red mottled). Mine also looks completely normal when I wake up. It does okay with no circulatory challenge such as standing long or leaving it down. Worse later in the day. It definitely seems like a circulatory, neurovascular interface thing. I have been told it will be the last thing to go, I just wish it would already. Seems to me like some discomfort would go with it!