As stated, he still needs to try having regular erections, "penile physio", even if they aren't particularly exciting.
While on hormone therapy, he should be able to get regular PDE5 inhibitors from the GP (Viagra/Sildenafil, Cialis/Tadalafil, etc) which may help with generating erections. Without libido, he probably still won't be able to have intercourse, but may be able to masturbate. He also will find it much more difficult to orgasm, although that's not required for "penile physio". If the tablets don't work, then regularly using a pump is highly recommended (again, may be available on NHS, but varies by area). If you can get them, it's thought to be worth taking Cialis/Tadalafil even if it doesn't work for erections, as it still helps improve blood flow in the penis and help preserve tissues.
There's no consistent evidence about Testosterone levels after adolescence changing with erections/sex. You can find claims that it raises it, lowers it, or doesn't impact it, and also that abstinence raises it. I would ignore any such claims as they are inconsistent, and any such change is unlikely to have any impact on prostate cancer.