You can bathe them pretty much whenever you just can't stand them anymore - like kids, lol. One thing that can prolong the time between baths is brushing with a boar bristle brush. It distributes the oils all along the hair and is really good for the condition of the coat, too.
As for the Frito smell, I have no idea. It may just be his peculiar body odor. The only other thing I can think of is possibly yeast. Try adding some yogurt (with active cultures) to his diet and see if it helps. You'll have to give it some time, though.