My daughter is doing very well in New York State Universal pre-K. 5 days a week, 930-12. Everything is predictable and structured. She doesn't resist going much, but she can articulate that if she doesn't care for it, it's because there's a lot of work to do, not enough playing. Not much I can do about that going foward, so I just philosophize about it with my daughter. She's going to Kindergarten next year so a little squishiness can't deter me.
In your case though, if I were in your shoes I would be concerned. I'd talk to teacher and your son and figure it out. If I couldn't figure it out, I would pull him and try again next year.
Danielle, Nick, Gigi ('08) and William ('11)