Good luck sev I hope he does well in the upcomming swimming meet here are some tips,swimming is a sport that requires both muscular strength and endurance, and for this reason when training with weights you need to concentrate on developing strong muscles with high endurance capabilities.
Aim for either high reps 15+with low medium size weights ideally for 1-2 sets, to build good muscular endurance, followed by 1 set using a heavier weight