You are a long way off from having to make the decision so you should be open to the development that will come this year. That said, I need to tell you that we did not send our son until age 6 - his birthday is in late June. Readiness has nothing to do with academics, in my mind - there are people who teach their kids their letters at age 2 and then scramble to find a private kindergarten to take a 4-1/2 year old. What's the rush?? If you are questioning his readiness, then I would say to hold him until he's 6 - unless something incredibly dramatic happens in the next 6 months. You cannot make a mistake by holding him a year, but if you start him and he turns out to not be ready, you will be faced with the issue of having him repeat a year while his friends move on.
Our son had gone to preschool for several years, so we put him in a pre-K program with a lunch option a few days a week. It was a perfect program and gave him a year to build skills and maturity. Our son was on the small size but I think that children's physical size is very deceiving anyway - you never know what will happen to them as the years pass and the other kids catch up.
Also, our son took a 3 hour nap well past the age of 4. There is no way he would have been able to handle a school day that went until 3 PM without another year of development.
Most towns have kindergarten screening by the teachers in the spring. This is for the purpose of assessing readiness. It's not the only tool you will use, but it's one of them. Another will be preschool recommendations to you. You could always have your child attend that screening, and then make a decision after speaking with the teacher. Your child doesn't have to know what he's going for - just say it's a quick play time and Mom will be there.
Good luck.