Other than the excellent advice given here so far, I would like to add that the focus point seems to be a bit more on the center of the image (his shirt) than on the face. I could be wrong and it's not so easy to tell because of the small resolution, but it seems like his face is a bit out of focus.
You have to remember that Pentax does not give you face or eye detection, which would make this job a bit easier. You can still get great shots as people here have proven time and again, but you have to be mindful that you are the one controlling where the camera is focusing. Me, I just use the old technique of "get focus confirmation on where you want to focus, then reframe for composition". If you did that and still the face was a bit off focus, then it's time to fine tune the AF of your lens.
The advice you got on the light challenges is right on, this is just a very difficult light situation. I've shot my kids birthday parties in very similar light and I know it's a challenge, but you learn to deal with it as you get more experience. Fill flash, as people have said, is your friend. Also, the composition advice (keep the distracting elements out, like the table edge) is very helpful. Composition is everything.
Other than that, it seems like you raised a good kid and you should be proud! And hopefully you should be able to have him and other family subjects around as willing subjects on other times, to take other portraits as you get better and better with photography