If you think it could be a focus issue, it might be ADHD. A lot of people think that if a kid with ADHD can focus on certain things, it's not ADHD, but that's not the case. DD could read well before kindergarten and was very focused with reading, or doing things like jigsaw puzzles, but other things, like drawing a picture or coloring--it didn't happen very often. Kids with ADHD are able to focus or hyperfocus on things that are interesting to them but have problems staying on task with simpler or more boring things (or things that they find too hard). She's also very "visual spatial".
If he's 5 he is probably too young to diagnose but it's something to watch in the next couple years. DD looked similar to her peers in K (and was therefore grade accelerated) but she looked unfocused in first and second grade compared to peers. There is a lot of change between kindergarten and first or second grade. "Normal" kids grow out of their focus issues, and ADHD kids do not.
In terms of the "triangle" example, I'm not really sure what to think of that, esp. if he could read. Both of my kids could name all the shapes at age 1.5 or 2. I think DS's first words were actually names of shapes, rather than "kitty" or "cookie"
