In colonization, bacteria have taken up residence in the respiratory tract, but are not multiplying rapidly, as they would in a infection. Not everyone with cf becomes colonized by Pseudomonas, but almost 30% have it by age 1, rising to 80% by adulthood. For various reasons having to do with the properties of the bacteria and the environment in the cf lung, Pseudomonas is really hard to get rid of once it has colonized the respiratory tract, although some people with recent colonization have responded sufficiently to aggressive TOBI treatment so that their cultures appear clear. Whether or not the Pseudomanas is really gone, or just reduced to a concentration too low to show up in the culture is not yet known -- nor do we know whether such aggressive treatment might eventually lead to TOBI-resistant bugs. I would be concerned about using TOBI just once a day; it would allow the concentration of the TOBI in the lung to fluctuate too much between doses, which could well increase the risk of antibiotic resistance.
Whether or not your child continues to harbor Pseudomonas, probably the best thing you can do to reduce the risks of outright infection is to make sure he gets good airway-clearance treatments every day. I don't know if the Vest is suited for kids as young as he is (can someone with an infant help here?), but if it is, I'd make every effort to get it.
As others have indicated, please know that the diagnosis of Pseudomonas colonization, while not a good thing (it does increase the likelihood that full-blown infections will take hold) is not by any means the end of the world. Our son also began to culture Pseudomonas when he was still an infant. We've been really careful about his respiratory treatments and have always encouraged lots of exercise, and so far it seems to have kept him in pretty good shape. He's 16 now, runs cross-country competetively, and can wear me and his dad down with his energy level any day.
Bambi, mom of Jordan