My heart goes out to you and those like you who have children with SCLS. The condition is bad enough for those of us who are adults, but at least we have a much shorter window of life expectancy than do children, all else being equal. Moreover, many of us have already buried our parents, so at least they don't have to keep worrying about us like you worry about your son.
A dozen years ago, the state of knowledge was such that SCLS was believed to be purely an adult condition that struck when one was in the late 40s or 50s, and I was a typical case with my first episode presenting just after I turned 56 years of age. However, now we know that SCLS afflicts people of all ages, including children, though men and women in middle age do constitute the majority of reported cases.
I now uploaded in the Disorder Resources section of this website the summary of a just-publshed article reviewing the evidence on SCLS in children, and it identified 32 cases that have been reported on in the literature worldwide. The article doesn't have answers for your questions, but if you or anybody else would like to receive it for background purposes, let me know (at firstname.lastname@example.org) and I will email it out forthwith.
Now, as to your questions, remember that while I've learned just about everything that is important to know about SCLS, and I keep myself very much up to date on the latest scientific and medical advances, I am not a physician and I am not otherwise licensed to provide medical advice. You ask "Mainly [my son's] symptoms are fatigue, severe headcahes, sore throat, blocked/runny nose, cramps, nausea, diahhoera. Are these symptoms of SCLS or Immunoglobulin therapy?" You should seek an answer from an immunologist who has long experience treating patients with immunoglobulins for all sorts of conditions, but in my modest opinion, there is a third possibility and it is that the symptoms you cite may be neither caused by SCLS nor by immunoglobulin therapy.
As to your other question, "His Dr. has suggested that they increase his SCig to see if it helps, but I am hesitant ... because I know that he will eventually have to go back on IVig and it made him very sick," I ask you to keep in mind two facts: (1) we still know very little about SCLS in children, so unless parents like you -- and adults like me -- are willing to experiment, we will never know what works and what doesn't; and (2) immunoglobuin therapy helps save millions of lives affected by various conditions every day -- including my life from SCLS -- so you should regard it as part of the solution rather than part of the problem if your son does have SCLS. True, some people experience some undesirable side effects -- I'm lucky and don't experience anything beyond occasional post-infusion headaches -- but the benefit is no doubt life-saving and quality-of-life improving for the overwhelming majority of patients who receive Ig therapy for their proven conditions, including SCLS.
Besides, children change rapidly as their minds and bodies evolve and adapt, so just because your son reportedly reacted badly to IVIG five years ago doesn't mean that he will do so again now that he is a teenager. Surely his body has gotten used to this blood product by now, so an increase in the dosage should not cause the same effects that it allegedly had when he was first exposed to IVIG -- and that's on the assumption that the IVIG, rather than something else, was the cause of his problems. However, if you are convinced that the extra immunoglobulins he is getting are responsible for the health issues he is experiencing, well, there is one sure way to find out: discuss with your doctor the possibility of suspending his treatment altogether to see what happens next. Maybe your son's problem is SCLS; maybe it is immunoglobulin therapy; but maybe it is something else entirely.