hmm, I've had lots of sparring with my stablemates and a few real fights back in the days, but I've actually never fought with a person who doesn't know at least some form of fighting techniques since I was a punk kid, the closest advice I can give you is that without rules, almost all fights end up with grabbing, and since he's taller than you you might want to get close to him anyway, I suggest you look up some Muay Thai grabbing techniques on the internet, if he throws a punch at you, try dodge it, get in, grab him by the back of the neck and start swing your elbows at his face.
here are some more dangerous street techniques, only use in case of emergency: put some bricks in your bag and use it to hit people, works great if you're in school uniform, my dad destroyed countless enemies with this method after school because they thought his bag was full of books like everyone else and chose not to dodge the incoming swing.
Three cans of soft drink in a plastic bag makes a perfect flail, but you might want to use two or three bags so it lasts longer, if you're in a hurry, some stones in a sock can also do the trick, but with smaller range.
A wet towel can hit as hard as a club if you swing it fast enough, carry one along with a couple of bottles of water with you.