Villain and I have virtually identical stacks. (~$350).

Villain is a white male in his late 20s. We've played at the same table a couple times but haven't tangled in any significant pots. He has a good understanding of the game and plenty of experience. He's a LAG whose VP$IP is probably at least 50%. He makes a fair number of small preflop raises but doesn't win a lot of pots uncontested, probably due to how active he is. He will also limp in with seemingly trashy hands and at least contemplate making ridiculous calls of preflop raises once he has limped. He's very active postflop and is not afraid to put money in without a strong hand. For example, he'll raise with a pair and the bottom end of a straight draw. He often gets caught making bluffs and semi-bluffs, although they're usually not for huge dollars. Also, he has the ability to take a stand without a big hand.

9-handed
MP open limps, CO raises to 15, I call in SB with TT, BB calls, MP calls.
Flop (60): QJT
I bet 50, BB folds, MP folds, CO calls.
Turn (160): 6
I bet 115, CO calls.
River (390): 7
I shove 160...

I feel that I played this hand well, although I'm not really sure what I would have done if the river had been an ace, king, 9, or spade.

Thoughts on that or anything else?

Interesting hand. Which range did you put him on when you got to the river? If he's getting there with mainly missed draws ( Asxs, KJ, T9, etc), and he has a high bluffing frequency, I wonder whether x/shove is more profitable than shove. Not sure if I really have enough info to speculate since the chance of that happening has a lot to do with dynamics between the both of you. Is there a chance he can see your range as a lot of medium strength hands or pairs + missed draws? Doesn't seem like it, but some players love to bluff (almost instinctually) when they get to this type of spot.

As far as hands we beat that get here and now call our shove...AA, KK, AQ, KQ. Since he closes the action HU I'm not sure whether he flats his overpair + hands or raises the flop. By the same token not sure if he's flatting or raising with hands that beat us OTF (AK, QQ, JJ, 98). It would seem like he would think he'd have to raise with a lot of his 2pr+ hands to both protect his equity and avoid an action freezer.

I had a bad feeling throughout the hand, like I was in the middle of a cooler and destined to go broke. However, as you say, it seems very risky for him not to raise the flop with AK, QQ, JJ, or QJ. I'd say tentatively-played (and perhaps rightfully so) AA or KK are in his range. Perhaps AJ or KJ, but again, those seem like prime candidates to pile the chips in earlier in the hand.

In my experience, it seems that in most cases when someone flops a monster and disguises it by calling the flop, they are ready to jam it in on the turn. This is probably because the pot is usually more conducive to getting all of the money in by that point. Also, if they get it in on the turn, they don't have to deal with scare cards on the river affecting their own hand or their opponent's.

To make a long story short, I'd really expect to hear from him on the turn if I'm behind.

It's pretty narrow, but I think the most logical range by the river is AA, KK, AQ, and KQ. It's probably wider, but I just can't make sense of it.

Result:

Villain tanks.

He says, "Why wouldn't you raise preflop with AK?"

That makes me a little nervous, because I'm getting MUBS and thinking about QQ and JJ.

Finally he makes a comment along the lines of, "Am I really going to make this call with middle pair?" He then briefly turns over AJ (not suited, didn't notice if either was a spade) and mucks.

I was flabbergasted. It's clear that he does not give an F about the money at these stakes and is willing to at least consider big calls if he trusts his read. Kudos, I guess.

He mentioned 99, but I'm not sure what else he thinks I could have, other than some other busted draw.

Were you playing a lot of pots right before this hand? Sounds like either a) villain thinks you semi bluff a lot, b) villain is afraid of any opponent "outplaying" him in every hand he plays, c) villain thinks everyone thinks like he does and therefore expects tons of semi bluffs on this board with your line.

He was being much more active than he was during our previous session.

I think the best answer of those choices is C.

He may be likely to make a play like mine with a combo draw, but I think it's safe to say that I almost never would. I think he's the kind of guy who's always suspicious that he's in a leveling war, and is inclined to make hero/speculative calls as a result.