When playing against an overweight opponent, there are a few things that you must remember.
#1 Don’t underestimate him based on his size
#2 Realize that he probably stays very close to the table
#3 Realize that he will likely have difficulty moving in-and-out
#4 Realize that he probably uses his wrist well and can be very deceptive with his shots
 
With these 4 things in mind, start the match seriously and be respectful.  If you mentally criticize your opponent because of his extra body weight, because of his strange clothes, or any other reason, you won’t be able to fully concentrate on your strategy.  Because he stays very close to the table, he will likely be able to cover the corners very well.  Sometimes hitting the ball deep toward his belly is a great strategy.  Surprisingly, he can probably reach and cover the angle balls fairly well, but he will likely have major problems when moving in-and-out.  Most of the short game play involves not only stepping forward, but leaning forward.  Without good core strength and posture, it will be very difficult for him to lean forward.  Start with a very very short no spin serve.   After he flips, try to attack quick to the middle.  Or start with a very very wide angled backspin serve, after he pushes, try to attack quick to the middle.  Also realize that he likely has other strengths to compensate for his lack of mobility.  Watch out for trick serves and deception in ball placement.
 
If you take this match seriously and apply these strategies, you will be well on your way to beating the heavy-weight champion!