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In my opinion this is a decent move against a decent/aggressive player. My guess is he had that type of read on you. So take it as a compliment, but don't fold here unless your read on him is very passive.

The features of this hand that make it a good spot to try this are:

1. Your pre-flop raise, which will be high cards more often than a pocket pair;
2. The low flop: there is only one card in the "playing zone" of 9 through Ace. This means you are likely not to have improved if you raised with big cards.
3. The fact that the pot is heads-up on the turn, but isn't completely tiny (I count 5.25 BBs after your turn bet, so I guess that is pretty small); and
4. He's got a ton of outs if you're ahead. The pair + OESD means he's got up to 13 outs, so it's a lot stronger than if all he had was the straight draw.

If the flop had been K J 4 instead of J 6 4 I wouldn't like this as much because there would be a higher chance that you had made a pair (of course, he wouldn't have had the straight draw then, but you know what I mean).

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I agree with the above + one more point - with his small pair, he was ahead of 2 overcards - even more reason to push a little in this spot