Sounds to me like your only real concern is that he still wets his pants sometimes and 5 months is a very long time when it comes to gaining better bladder control. If the preschool teacher had concerns they should have expressed them to your by know. If he is bored at home now then I would think that he is ready.
I think you need to remember that the prep/ preschool year is designed to get kids ready for real school. It is learning the very basics and lots of play and social skills. No matter what age they make the first year of school there will be kids who come from backgrounds where they have had no educational influences. So it has to be really basic. I second going to check out the school and facilities. I went for prep open day yesterday and it is really just like the preschool room that my DS has been in at daycare. DS didn't want to leave as the playgrounds were so great.