He couldn't keep up with his friends, and eventually fell behind.
Literal
Friend [target-に] follow [potential-ことができなかった] [because-ので], finally he [topic-は], fell-behind [regret-しまった].
ついていく ('to follow along, keep up with') uses ~ていく for motion away from the starting point. とうとう ('finally, in the end') often accompanies negative or unfortunate outcomes, conveying inevitability. ~てしまった marks the falling behind as regrettable or beyond control.