Is he sparring, or is this a serious fight?  If sparring, take a look at some martial arts videos on YouTube, and see how the multiple-attack training katas or free-spar sessions look.  People who are attacking take turns at the defender, they broadcast their intentions clearly, and they cycle through the attacks from various points instead of all attacking at once from everywhere.

If it's a serious fight, then there are harder choices to make.

Does he HAVE to win?  What does winning mean?  Does he simply get away from them?  Does he have to defeat them utterly?  Does he simply make it so much trouble and pain to get him that they back off because no one wants to mess with the crazy person?

With all that in mind, here are a few concrete suggestions, aimed at a believable ACTUAL fight.

Suggestion 1 - have him outrun at least 2 of the 4 dudes before the actual fight.  Better if he's able to outrun/outmaneuver at least 3 - maybe after running from 2 of them, he ducks into an alley which is so narrow that only one person can fit through at a time. 

Instant one-on-one for the fighting, but still has the power of the multiple attack - the suspense of the chase "Oh good, he finally lost Goon 2!  Oh no, Goon 1 has come around the block on a motorcycle and is gunning for him again!"  and then the suspense of the actual fight  "Will he have enough strength left to fight Goon 3 after running so hard?  Yay!  He beat Goon 3!  But uh-oh, now Goon 4 is climbing over his buddy's body and coming in again - now what?" 

Suggestion 2 - Find some reason for the bad guys to seriously underestimate his abilities from the start, so they are caught off-guard when he actually turns and fights.  That underestimation makes fighting two opponents a little more realistic. 

(Very rarely, even with trained martial artists, does one person win against multiple attackers, and when it happens, it's usually because the groups or pairs of attackers weren't really trying their utmost from the start because they outnumbered the victim.  Expand and elaborate on their superiority (in their own minds) and then use that overconfidence against them.)

Suggestion 3 - Take a serious thought at the concept of berserkers.  There's a reason Wolverine is named after that particular animal, and it isn't because of its SIZE.  Not many people want to tangle with something crazy.  That's a deep primal instinct people have to steer clear of odd behaviour. 

Think of rabid animals.  A rabid rat isn't that big, and rats usually run from humans.  But if this rat's gone round the bend, it will attack, and that's not NORMAL for that small thing.  If tiny dude is freaking out and throwing himself into the fight with everything he's got, that makes the Goons wonder; "oookaaay, what is tiny dude ON, and why is he growling at me?"  These are not confidence-inspiring questions.

Whatever mechanics and improvised weapons you use then will be more believable automatically, because the SITUATION they're in is more believable.  Because of the setup, people will more readily keep on following you down the "Hey, did you know keys are a kickass improvised weapon?" path, regardless of the details.


Good luck!   

This seems to be pretty unusual in fiction, almost an inversion, in fact.  One way this might be handled is to have the character uninterested in building a relationship with his/her biological parents, but seek them out to see if he/she has any inherited medical conditions.  Then the parents turn out to take an unusual interest in the protagonist...