Originally Posted by Boxed
How often are you going to the boxing gym? If your training 3 times a week any additional weight work will be beneficial.. I wouldn't worry about bulking up and slowing down; this will only happen if you do extremely heavy weights, and evidently you're not if your benching 20kg..
If he increases his max strength he can increase his explosivity.
OP I would say increase your max strength. Do squat, deadlift and bench with heavy weight 3x5.
Someone more knowledgeable will probably give there 2cents, so listen to them over me.