Here is what I do with someone with backspin. This is going to be more effective in high school then it would as you move up the ladder in your rating and play better and better players.
But my strategy is always the same just may require a little more patience playing against someone with a good slice.
The slicer has a tendency to either float long or leave the ball short in the court. I play a friend of mine with an excellent slice - I mean an excellent slice. However, he has a tendency to hit it short. I am always ready to move in to take that ball as an approach shot. The approach shot is sliced right back to his backhand as he will probably slice again and the ball will be coming up. A volleyers dream.
When he realizes that his slice is being handled and it is know longer a weapon. I pump my fist up and shout, COME ON!!!!! Just kidding!
Also, I will serve 90% of my serves to his backhand slice. It provides me with a lot of time to get in and volley the ball.
The shot you hit in a rally especially since your learning fundamentals, is to return a slice with a slice. YOu can also learn about Wardlaws Directionals so you know hoa to play the percentage shot.
Also, the slice shot is a feel shot, so be sure to mix up your speeds, spins, hieghts and depths of your shots. You might find he has a great slice at waist level but leaves the ball short if he has to hit the ball shoulder high and above.