I'm another one who had these concerns in K and turned out to be right. I fought and fought to try to get an evaluation but the school kept saying "She'd never qualify for services - she's too smart." When I specifically asked about the possibility of dyslexia the literacy specialist refused to make eye contact and insisted on changing the subject. We knew DD had a fine motor deficit and had her in private OT but I was not aware enough to force the issue of dysgraphia.
The instant the school psych from our home district saw a sample of DD's writing she agreed to do a spec ed assessment. The inter-district magnet she was attending was still refusing so it really depends on the policies in your specific school as to whether or not they will follow your "mom gut" and evaluate. Personally I would start asking about it now since it could take some time to get the school on board. The issue will either resolve itself or you will be that much further along in the process of getting them to test.
And yes have his vision checked. Like syoblrig my DD also was an early talker with a huge vocabulary. She has now been identified with just about every possible LD there is so you just never know...
Oh - and just say "No" to the extra writing for homework. Personally I'm opposed to the idea of homework in K but if they send it he really shouldn't have to spend more than 10-15 minutes at most total. If he cannot do it in class they need to address why, not make him do it at home.
