Had prostate seed implant surgery 6 months ago, wearing an urine catheter ever since, can't urinate on his own. what could be the problem?

See below: It could be a stricture or scar in the urethra due to effects from radiation, or it could be bladder dysfunction from another cause. Please see a urologist to help figure out what is the matter. Good luck.

Obstruction: Your prostate should have been evaluated for benign prostate enlargement prior to seed implants. Check with your urologist. Conservative surgery to relieve obstruction can be offered 6 months post implant.

Brachytherapy: Prolonged urinary retention following seed implant is very uncommon. I would consider seeing another urologist if the best option your treating urologist can offer you is a Foley catheter 6 months after treatment.