Re: Would you put him in PreK?
I would have no reservations about putting him in, even with all of the little potential bumps (sister, naptime, age variations) as long as he was comfortable with being away from you. My daughter is a pretty independent person, but at 3 she wasn't ready to be away. At 4, she leaves me behind without a second glance.
Your son will adapt as long as he feels secure.
I run the Frontier Wholesale co-op in February, May, August & November