In my experience when I was having episodes of SCLS (late 2005 to late 2009), the symptoms sometimes were the commonly reported ones (runny nose, flu-like, or stomach problems) but sometimes they were not: towards the end, I had no early warnings at all that an episode was underway -- and I was getting them more and more often.
Therefore, my smart doctors got me a HemoCue device so that I could monitor my hemoconcentration on a daily or half-daily or even hourly basis -- just like diabetics check their sugar levels to see how they're doing. (There is no easy way for a patient to check his/her albumin, so that is not a practical alternative.) That way, I would know with prompt and scientific certainty what was going on inside my body, I could phone or email my doctors about it, and we could do something early on -- before I passed out -- whenever the readings were abnormal.
So I started to monitor my Hgb (hemoglobin) and to keep detailed records of the results, and did so for over a year during which I had episodes (six months) and also after I stopped having episodes (another six months), thanks to monthly IVIG infusions. I have stopped measuring myself since mid-2010, because after going on IVIG, my readings were absolutely normal day after day, month after month -- so there was no point in pricking myself.
The monitoring of my hemoglobin for over a year provided a treasure of clinical information about SCLS. For example, while at the beginning we would take every upward departure from my normal hemoglobin range as the start of an episode of SCLS warranting a medical response and hospitalization, we discovered that there are cycles of hemoconcentration that reverse themselves on their own before reaching clinically significant levels, so they can be managed at home by taking Prednisone or nothing at all, depending on the HemoCue readings.
This is consistent with reports from other patients at an advanced or uncontrolled stage of SCLS of frequent, self-reversing cycles of swelling, limb pain or dizziness that they endure without requiring hospitalization. Therefore, we came to redefine what constitutes an episode of SCLS warranting hospitalization and intravenous medications -- in my case, as one where the hemoconcentration reached 20 gr/dL. (The trigger would be much lower for a child, of course.)
Thus, my recommendation is that, if you really want to know what is going on with your son -- when he has a cold but not an episode of SCLS, or when he has an episode but no cold, or when he has a mild episode that can be monitored and treated at home, or a severe one requiring hospitalization and IV medications -- then get yourself a HemoCue and start monitoring his Hgb every day, or twice a day, and more often whenever the instantaneous results you get are higher than normal.
Yes, you do need to draw a big drop of blood with a lancet each time, and it does hurt, but kids and adults get used to everything. Your family will finally have peace of mind, and you will taking him to the hospital only when he really needs to go there. Think about it: no more wasted hospital visits "for observation" like the one you are describing -- and, at the other extreme, no more instances of keeping your son in school or at home when he _should be_ in the hospital.