Very often the bike paths along a river are crushed stone, converted from barge towpaths or railroad beds. If this is the case, look into a hybrid bike. These paths are typically flat and smooth. He won't need knobby tires for traction or a suspension for bumps. Those things are unnecessary weight and complication.
The advice you got to go to a bike shop is spot-on. Don't go to a big box store or even a place like REI or DIck's. The people at the bike shop are trained to help your BF select the right bike and then fit it to him. That makes a HUGE difference in how much he'll enjoy the bike.
You're a good GF to buy him a bike!
HTH