Any advice for my boyfriend?
He has a six pack and nice chest, but his arms and legs need improvements.
Its quite simple, he needs an arm day. Does he regularly workout? If so, he just needs to add intensity to his arm workouts. 1-2 bicep exercises (curls, pullups and their variations) and 1-2 tricep exercises (pushups are great for beginners, and dumbbell extensions or bench dips)
Secondly, it would benefit his strength to do a forearm exercise and work his shoulders (military presses and laterals)