Analyzing HU hands is very tough. So much, IMO, has to do with feel. Momentum, changes in betting patterns, aggression... and a handfull of other factors can dictate play and are difficult to incorporate in a hand analysis.
You had been aggressive and him passive, allowing you to win pots with any cards. You had a "feel" that he would soon take a stand to slow you down. Well, on this hand he may be doing that... just playing aggressive with 2 overs or worse. But he may have actually picked up a hand. I don't stake my tourney life on a guess that he's wised up to playing aggressive (but can't beat a pair of 5s).
I probably call the flop. Its not a lot of chips and it says, "hey buddy, I'm not sure what you're doing. But if you think you can bet at any flop and take it (like I do), then you are mistaken. I think 500 chips can be sacrifised to make this point.
If he comes out firing on the turn, I let it go. I don't want to lose 20%+ of my stack on a pair of fives. But he now feels that he needs to fire 2 bullets to push you off a hand (or bet the flop harder). Perhaps this will allow you to trap him better if/when you flop a monster.

Fold or Push. As the short stack you dont have much more choices.

Oh, just a little observation :

In Heads Up I NEVER call as SB. Its either raise or fold

Giving the BB a chance to see a free flop an outrageous mistake ( well I think though I see lot of people do that)

On the flop, do you think your middle pair is good ? If yes you push, If no you fold. You cant play much post-flop HU when you short stacked.

He can have anything

HU, I raise every button, and a lot of hands in the BB as well. You need to get your opponent to define his hand quickly . . . .

Particularly when you're so outstacked and short (low M) . . . . you've got to be aggressive here.

If you feel like he's going to start playing back, then it's fight or flight time. If you don't like the hand, fold PF. It's not a crime to fold this hand here.

If you're going to fight, you need to really unload on the flop, not just do a min raise. I'd dump my whole stack in (a call or weak raise commits you to the pot anyways, so just fire everything you've got now while you likely still have outs even if you're behind). . . .He's only 2:1 against to hit the flop, and much worse to have TP. You may be way ahead here.

Once he's called the flop raise, you're all in or fold on the turn (again, look at the stack sizes and the blinds and the size of the pot).