Looks like the other posters covered it, the public access areas are well known and generally very well marked. Also, if there isn't a fly shop in Granby there is a tackle shop there that should be able to provide some current lake and river information as well.
Given the low flows this year I would not plan on fishing the Colorado in RMNP. It's a smaller stream/creek and my guess is that by August it will be a trickle with stressed, spooky fish. Lower down should be a lot better, as Sandan said hot sulfur springs or below.
The good thing about that area is you have options. The lakes hold plenty of fish, generally they are bigger than in the streams, and if you catch a good hatch can be a blast, but if you are set on river/stream fishing then the middle Colorado river would be a good place to check out.
Guided trips are also another option that help take away a lot of the guesswork and can get you into private water that doesn't see quite the pressure the public places do. Some shops offer float trips that are about the same cost as wade trips...floating is a fun way to fish (though be sure to ask about spin fishing if your son doesn't want to fly fish). Some guides or maybe shops, I have no idea, only allow fly fishing according to others who have inquired.
Good luck! Let us know how you do.