You can do an online search of the kindergarten requirements in your state online. You can even find information about the standards for the kindergarten year (what they are supposed to know by the end of K). That is if your only concern is academic.
I've worked with 4-5 year olds for 11 years and I own one this year myself. I've found that boys in general get interested in writing/drawing later than girls, almost always after they are already 5, and that it will happen very quickly like a switch. Just bam, and one day he will want to write and draw all over the place.
They all eventually get the academic stuff. Here in Massachusetts K isn't even required by law, the kids just have to meet the academic standards and age requirements for first grade. So as long as that stuff is learned before first, they are good.
The concern shouldn't be academic at all. In all my 11 years I've only seen one kid that I recommended repeat the K year because of his academic skills. The concern should be social emotional.
Is your son confident enough to be in a room full of strange children and still make friends without you having his back? Is he tempered well that he won't yell or fly off the handle at the first sign of a disagreement? Is he beginning to try and solve social conflicts by himself and through talking, reasoning and cooperating? Does he finish what he starts? Does he become easily frustrated? These are all things to look at when considering whether or not to hold a child for a year or send him to K right when they meet the age requirement.
I don't know your boy, so I can't judge him or give you an opinion about him, just generalities I've seen over the years. Most boys who are 5 after June benefit greatly from an extra year. I've seen both situations and lots of permutations. Some go when they are 5 and do great, others go when they are 5 and have a very hard time. I've never seen a problem with a kid that's waited, they have all done well.
I hope some of that helped.