Shirley,
In order for a mortal sin to be committed, it must be done freely and knowingly, in addition to being serious in matter. If he is not free to go to church because he has no transportation (and walking or biking is not feasible), he cannot commit a mortal sin.
If he refused an opportunity for alternative transportation without good reason, however, he might be held fully responsible. If you live nearby, you might consider offering him a ride or facilitating alternative transportation.
Even if it is not a mortal sin, he is at a disadvantage not going to Mass.
Eric |