he's probably just being 5!
i woudln't worry too much. maybe singing and stuff isn't really his thing. maybe his spirit guides him to question the world and nature and what's going on around him more than just being satiated by singing in a group.
example:
our dad was a big game hunter when we were growing up and instead of paying attention to the butchering of the deer one day, my sister was poking the deer's eye with a stick and told my dad "hey, this deer's eye is really tough! i keep poking it and it's not doing anything!"
sorry, it's a bit graphic, but my point......my sister is now an RN with her bachelor's in science and has recently gotten into administration at a large, magnet medical center in NC.
so, sometimes kids follow their own internal voices as to what guides them and what interests them.....
so, it just sounds like he isn't always interested in what "other" kids are interested in...nothing wrong with that! he could be the next great philosopher or scientist or even, mathematician.
give him time...he may start to pay more attention as things become more interesting to him.
another story:
a guy in my class in high school always received Ds and Fs in class b/c he never did his homework or assignments. however, at test time he would get straight 100s. the reason....he just wasn't interested in the class b/c it was so beneath his level.
he ended up going to an ivy league school b/c his SATs were remarkable. once those schools realized his talent they couldn't wait to scoop him up!
so, hang in there and find out what does interest him....maybe nature holds his attention. try going for a "nature" walk in your backyard. flip over rocks to see the bugs or look at leaves through a magnifying glass.
take care and don't fret...he just sounds 5 to me:)