My DD was not early-entered into K, but she did exhibit some of the same behaviors. She hated school, and said she had no friends there. The latter was confirmed by school staff, who described her as quiet and compliant. We knew our DD as compliant, but quiet is only in character for her in new situations, as she's somewhat guarded, but she should have come out of her shell at some point within the first several weeks. DD was also having meltdowns at home.
In her case, the problem was that the instructional time involved materials which were significantly beneath her. She was bored, frustrated, but also trying to fit in. The tension between seeking to conform and seeking something to learn was wearing her out, and causing all of her social and emotional problems.
In that situation, holding her back another year would have been a disastrous move, as it would have only made the problem worse. What she needed to eventually solve the problem was the opposite: a grade skip. Until we got one, we homeschooled our DD, and saw improvements almost immediately.
I gather from your description that homeschooling is not an option. An alternative may be to go ahead and pull him out, send him to the fun daycare he loved, and then send him back to school next year... in first grade.
