He's probably working for ARC, ships that I know well. Thermal base layer, top and bottom is an absolute must, something that will fit under his coveralls (Ship will provide one set of coveralls, only to be worn in port as a uniform item). A good Carheartt jacket with a hood, as well as a knit cap and a neck gator that you can pull up over your nose. Carheartt makes a cap that has a neck piece built in, you can tuck the neck part in to the hood if you're not using it. Gloves that are insulated as well as work gloves, I'll usually have a pair for keeping my hands warm and a different pair for when I need to handle lines. Rain gear and water protection isn't super critical on a car ship, because you almost always have a roof over your head, even when you're out on the mooring deck.
Weather wise, the absolute worst part of the trade is the locks in Bremerhaven and Antwerp. Bow Lock to Stern Lock can easily take an hour or more in Belgium because they frequently will put the big car ship in the locks first, and then load in a small coaster or two, the coasters have to leave first, and then you start dropping lines on the car ship. All said and done it could be four hours from when you have to go outside for mooring ops until you're all fast. Temps in the low 30s and rain isn't uncommon in Bremerhaven. Standing in the wet and cold and miserable in the winter can be tough, but again, good Carhartt Jacket with hood, knit cap, something for your neck/nose/face, thermals, and gloves and you'll be set. I usually wear low cut steel toe keens with good socks and that is plenty as far as boots.
In general, I show up with three sets of Carhartt pants, a bunch of Carhartt shirts, as well as some sets of casual shorts and t-shirts for standing watch. A stiff belt, from Bison or Blue Force Gear can be useful because you'll be carrying a radio around. Most bridges will be pretty casual as far as standing watch, but don't look like trash either. A pair of shoes you can slip in and out of is nice, for walking around the house when you're not on watch, as well as a set of athletic shoes for going to the gym or standing bridge watch. I keep a golf shirt or two with me for going out and traveling, but don't otherwise wear it.
For an early Christmas get your kid a Spyderco H1, Streamlight Protac 1L-1AA, and a Petzl Tactikka+ RGB. Gold standard for knifes and lights.
When you have enough posts to PM, shoot me a message with the ship name and I can tell you whatever you want to know about it. ARC is a good place to cut your teeth as a deck cadet, interesting mix of ro/ro, breakbulk, and vehicle cargos, ocean crossings combined with maneuvering in busy traffic areas and good ports to go explore. Antwerp is a personal favorite of mine, and while it can be expensive to get in to town if you take a cab, there is a water taxi that goes from the locks at Kallousluis and will drop you in downtown for a Euro or two. Southampton is an easy walk in to town, and Bremerhaven is interesting as well. They occasionally add a European port or two like Zeebrugge or Gdansk which can be interesting.