All the best.
In this context he has three options available for him: 1. Active surveillance - they will follow his PSA roughly every 6 months to make sure he remains at low risk of it causing him problems, and then treating him when/if the time comes. 2. Surgery with radical prostatectomy. 3. Radiation therapy.
Within the realm of radiation therapy there are two basic types. The most common type is "External Beam". This is where the patient lays on a table and radiation is delivered by a machine that moves around him to deliver radiation. The radiation is very much like an X-ray in that the patient does not see or feel anything during the treatment. The catch here is that for prostate cancer, external beam radiation is delivered daily, Monday thru Friday, for about 9 weeks. The other type, the kind you are talking about, is called "Brachytherapy". This is where there is an actual source of radiation placed inside the patient to deliver the treatment. You can kind of think of brachytherapy as treatment "from the inside out" versus "from the outside in" as would be the case in external beam treatments. The "seeds" you mention are actual pieces of radioactive material that are placed in the prostate to treat the cancer. This is typically done in the operating room under anesthesia and the Radiation Oncologist puts many needles into the prostate and drops the radioactive "seeds" into place. Once the seeds are in place, they give off their radiation around a very small area over a period of time and eventually decay to where they no longer give off any meaningful radiation. These seeds are permanent and left in the prostate. Here the treatment (at least from your uncle's involvement) is really only one day, rather than the roughly 9 weeks of external beam.
Each of the radiation treatment options has it's own set of side effects that is beyond the scope of this reply, but it is important to talk to the Radiation Oncologist about the pros and cons of the two treatments.
and deliver radiation from the inside out. Radiation, whether from seeds,
or delivered by the more common method, from outside in, is very effective.
The five year relative survival for your father should be around 99%.