Is the first semester a period of adjustment? It seems like most of the freshmen at my sons school are on a performence roller coaster. Some days they do well, others they don't. Is this typical for freshmen, they need a semester to adjust to the college life?

Yes it is, and sometimes one needs more than one semester.

It gets better.

For baseball players:

Living away from home.
Being on their own.
Being away from or breaking up with HS girl friend.
Making new friends.
6 am weights.
Adjusting to college classes vs High school classes
Working out for 3-4 hours each day 6 days a week.
Trying to win a position on a new team.
Playing against guys who may be 2-4 years older than he is.
Going to the trainers 1 if not 2 times a day.
Living in the Dorms with what you can say is a party every night during the week even more on the weekends.

Not a lot to adjust to, at a 4 year school, for the first semester...

Even with non-athletes, often the first semester's grades will be the worst of the entire four years. So many adjustments to make. So much new to experience.

I am a huge supporter of a 5 year program for all athletes with the initial year being a non play-get acclimated year---the player settles in and at the same time can get 30 plus credits out of the way---something like a mandatory redshirt for freshman--the frosh get younger and younger each year and thus in many ways less mature

My son's only adjustment was to the marking system. He loved the whole change. Hanging out with teammates, practces, training etc. He was a little unnerved by missing classes which he had never done before. I never heard him complain and was really enjoying being on his own.