I wouldn't worry about it and wouldn't say anything to anyone yet. This is the first week of kindergarten - he's getting used to a new routine, a new set of classmates, a new teacher, a whole new reality. It sounds like he's having fun - honestly, to me that's the most important thing right now, this week, this month, for a kid starting school. Working on something simple that he already knows how to do - right now - isn't going to set him back and isn't going to keep him out of TAG when he's tested in February.
I'm also guessing that he's not the only kindergartner in his class who's reading leveled books (there were a few in each of my kids classes since so many kids go to preschool before kindy here)... and I'm also guessing that his teacher will figure out where he's at. The first week of school is just beyond crazy - and that letter work may have been a simple exercise given out by his teacher just to see where the various kids are at. Hs teacher will know a lot more about your son a few weeks in, your son will start showing more of what he knows a few weeks in once the newness of school has passed, and you'll know a lot more about how the class works etc and whether or not your ds is being challenged. It's just too early to know right now, and I'm guessing the TAG coordinator (as well as the teachers) are busy with beginning of the year work. If you do find you need to go in and talk to the TAG coordinator, it's going to be a lot easier to not sound like a helicoptering eager pushy mom if you go in with some accumulated work samples from school and home.
Best wishes,
polarbear
