Sure, its possible his hypothy will become permanent- but its not probable.
Very few cats <5%) become permanently hypothy after I-131 tx- and that's
usually because the dose was too high.

I'll explain how I-131 works so you'll understand what's happening:  The
I-131 is almost entirely concentrated in the hyperfunctioning (adenomatous)
thyroid tissue.  The normal thyroid tissue is atrophied because it hasn't
been receiving TSH stimulation.  Because the normal thyroid tissue is
suppressed, it only receives a very small dose of  I-131.  However, if the
cat received a large dose of I-131- the normal thyroid tissue receives a
higher dose of  I-131, too.  The more I-131 the normal tissue receives, the
longer it takes to begin functioning normally.  If the initial dose of I-131
was too high- normal thyroid tissue can be destroyed too.

As far as 3 months being a long time to remain hypothy- I can't say for sure
because the hypothy period is relative to the dose of I-131 and I don't know
how high your cat's dose was.  If he received a high dose- >10 mCi, then no,
3 months isn't a long time.  T4 concentrations in the blood are usually
normal within 2 weeks of I-131 tx in about 70 to 80% of cats, and in over
90% of cats by 3 months.  But as I said, a high dose can prolong the period
of hypothy or sometimes destroy normal tissue, too. The only thing you can
do is have him checked every few weeks and wait it out.   If he develops
clinical signs of hypothy, call your vet immediately and don't wait for his
next scheduled re-check.

Sorry I can't be more helpful- this is one of those things that doesn't have
a clear cut answer.

Best of luck,

Phil.