Wow, cant wait to see how well this plays out. One thing Id imagine you'd want to avoid is getting taken down. Im sure he'd prefer to wail on you while having you pinned. Sounds like you're relatively thin so you gotta be quick and stay away from him grabbing you.

excellent suggestion as well. Especially few good low kicks to the legs should deplete his stamina real fast

on the topic of kicks, watch this video

these kicks are not hard to immitate either so you should be able to pull them off

look at the video I posted, they won't be too hard to learn. Do the while moving around a great deal and the man will be tired real real fast..then you can just go crazy..elbows and knees are also very useful if the opponent is stronger than you physically. An elbow or a knee to the solar plexus or face could pretty much end the fight right then and there

Weakspots for fights:

Quick punch to the throat
Knee to the side of the kneecap (important, it's not a kick-type knee, you put all your weight into it and press down)
purple nurple (especially with fatties) Really any skin attack hurts with fatties. Getting a nice claw grip into the smoother fatty skin and squeezing it really hard is good. Back, chest, neck, rib cage.
clock to the ear/temple (clock is hitting with the open part of your fist, where the fingers show.
elbow to really anywhere (lower back especially, and top of head)

Too bad you don't know aikido, that's like made for fighting bigger people.
Fighting bigger people you need to control his movements and balance. Look up some aikido videos. Don't study the techniques, but take a look at how they stand. You stand with one arm kind of out, because what you're doing is baiting them to grab it, then you switch your stance, grab his wrist with your free one, step backwards, and suddenly you're in complete control.
Don't really try this kind of moves though, just keep in mind that you're trying to bait him.
Bigger guys are stronger, and you need to use their own weight against them to really make an impact.

Muscle mass increases metabolism and burns calories, thus preventing one from becoming fat in the first place. Most fat people are not as strong as you would think, especially in terms of upper body strength. The best advice I can give you is to fight aggressively as you likely have a physique more suited to fighting. Also, if he tries to get pushy, redirect his own momentum and throw him. Get some help with basic throwing and grappling if possible (watching UFC does not count, sorry). Remember to strike fast, hooks and uppercuts are slow and leave you vulnerable. Use jabs, crosses, knee strikes and elbow strikes. Practice your striking before the fight to help yourself become comfortable with these techniques. This will have the added benefit of allowing you to increase your speed and build muscle memory. No dietary advice is necessary at this point, the fight is too close to worry about that. The only thing I can recommend as far as what you eat is to consume a fairly light meal before the fight consisting of eggs, toast and plenty of fresh fruit. This should provide the simple sugars and carbohydrates needed for the fight. Good luck, remember that you have an advantage that if properly utilized, will allow you to attain victory.