Lisa, unless it's a severe case, I wouldn't worry about it (the circumcision, that is). I think your son may be just fine either way. I've had a mild case of ichthyosis all my life. Using AmLactin it's almost unnoticeable. But left untreated parts of my body are affected greatly, including my legs, arms, feet, hands, sides, and back. However, parts of my body are not affected at all, including my face, palms, soles, chest, and yes, my privates. It seems unlikely to me that your son will have to deal with a buildup of scales near the foreskin, unless you are already observing this taking place. However, if what Keith described has actually happened to him, clearly it's possible to experience discomfort in this area due to development of scales.
I am not that informed about the different classifications of ichthyosis, but from what I understand, there are certain parts of the body that aren't affected except in the most extreme cases. I would try two things:
1. Communicate with other men on this list who have had extreme cases and ask each one if circumcision made (or would've made) a difference for him.
2. Ask a doctor to do his homework (i.e. some research) and talk to another doctor who's seen a severe case of ichthyosis up close and find out whether ichthyosis is a good medical impetus for circumcision.
Regarding the appearance, trust me, most men are proud of what they've got regardless of whether they are circumcised or not. Much more important than whether they've got a tiny flap of skin is whether they are raised to be happy, confident, and self-assured.
I'm circumcised and don't have any ill-effects or miss my foreskin. I also don't have any scary memories of it happening. Most likely, unless you observe it, your son's ichthyosis will not affect that area. Unless it's causing him some discomfort now (in which case you shouldn't hesitate to do something about it), I see no reason not to wait and keep and eye on it while you gather more information.
P.S. I can relate to your concerns as a Mom. My mom used to get scowled at by strangers who thought she didn't keep her baby clean. It wasn't dirt, it was just scales from the ichthyosis. I hope your son is as lucky as I have been to find a treatment to keep it under control.