The erection nerves are in the form of a cobweb which isn't very visible during the op, and there's quite some variation in exactly where they are between different men. So occasionally when the surgeon thinks they were taken, they weren't all.
Given this, he should be doing rehab as for nerve sparing surgery, i.e. on regular Tadalafil to help with nerve healing. Sometimes this is done with 5mg daily low dose Tadalafil, or sometimes 2 x 20mg/week Tadalafil. The latter option gives you a couple of extra boosts a week to try for erections, as the daily low dose is more for healing and usually not sufficient for erections, whereas the 2 x 20mg/week will also give the continuous low dose because the drug is quite long acting, but also gives you a couple of boosted periods.
If that doesn't enable him to get regular good quality erections, he should ask for a pump too, to ensure he is getting regular erections to reduce damage to the penis (which requires regular erections).