I don't know how I missed the schizophrenia post...

...but, no, I wouldn't worry about that based only on the descriptions we've heard so far. Quite a few math-minded children perceive numbers as having other qualities. Synesthesia is not particularly unusual (I think I've mentioned that one of my siblings, now a math professor, associates them with colors).

I also would tend to downgrade ASD concerns based on the shifting of his obsessions, and how they correspond with new skill areas. Most children--gifted and otherwise--have periods of focus on mastery of a skill, or working through a developmental problem. It may be more pronounced in GT kids.

...pronounced like the long vowel and first letter of the alphabet...