He is ok. The rule applies to the vice versa.
In other words, if he ran unattached in a college race and then wore his jersey at the road race he would ruin his redshirt season. Even with that he could still sign up as unattached for the road race and then things get complicated. It falls back on the school to allow this or not. Some schools are protective and want their athletes to only represent the school. Others are less protective.
Long story short, there is nothing wrong with running a road race mid season. The only con is how will this affect his indoor training?