hi, if bf has problems he should if possible ask at the clinic or dr
testicles move around to maintain temperature so when hot the scrotum expands greatly and thins so to cool and when cold it shrinks drastically so to maintain warmth
from the little I can discern from your description it is perfectly normal because when he gets erected DOES IT NOT GO AWAY and if so, then it is normal and nothing to worry about
good luck to both and he is lucky u care so much