That is hard...
I also have the August 31st cutoff - and three daughters with summer birthdays, one who just makes it by a couple of days. I basically believe that if they make it, send 'em; if they don't, wait. I REALLY don't think anyone should test in for kids that just miss the cutoff but that is just me!
My dd1 (July birthday) went because she made the cutoff (had had two years of preschool already as is standard where I live). She did great in preschool, separated fine, loved it. The first few months of kindergarten were ROUGH - she is very bright with a great attention span but the young fives are just not as mature as the older ones and you can't rush maturity! She would get tired and frustrated by the end of the day. Granted, we had to do full day kindergarten (which I DO NOT RECOMMEND!!!!!! - for anyone!) and it did mellow out by January but it was a rough few months.
Now even given that, I will be sending dd2 (end of August birthday) to kindergarten "on time". Why? Because I've seen dd1 go through 1st, 2nd, 3rd grades now. She is in the right class, doing well, just the right spot for her. I can't imagine her in the class below - she would be bored.
The fact that you have to fight him to go to school half the time is a bit worrisome but unless he has some other issues that you didn't mention that would mitigate the situation, if it were me, I'd send him on time. Someone has to be youngest and someone has to be oldest. I think the August cutoff is good (CANNOT understand the states that have December cutoffs!) and given normal circumstances, I'd send 'em if they made the cutoff. I am STRONGLY against testing in the fall birthday kids after seeing my July birthday have a hard time - but I suppose everyone is different so I probably shouldn't say that.
If you do send him, do half day! Does he have any neighborhood buddies going? Is there a kindie open house when they do the registration - you could take him and feel it out? Can you request a teacher - maybe get recommendations from your friends with kids who have gone through?
Oh, and my dd2 (late August birthday - in her second year of preschool now) is not too interested in writing yet. She does write her name and once in a while her sisters' names if I help but mostly draws pictures. I don't think that his lack of current interest in writing means he isn't ready for kindergarten.
Also, I really think kids pick up on our emotions regarding stuff like this. If you are worried and unsure then he will feel that and wonder if it is safe or right for him to be there. I loved school, every minute from kindergarten through college graduation, so my vibes were super positive and my kids have loved school and not had many issues.
You asked if there are any cons to being six in kindergarten. Well, the September birthday kids (who are old in the class naturally) are six within the first week or two. Maybe they are noticeably taller but no big deal there. They obviously have a maturity advantage. Only con I can think of is maybe the kids wondering if he had to repeat a grade since he'll be older than the supposed oldest in the class. The summer birthday kids are always youngest.
I don't think I was any help.... Good luck deciding!