I don't see anterior femoral glide syndrome as a likely candidate just because he was able to deadlift conventionally for awhile without any issues. Conventional deadlift hits the hamstrings hard and if the femur is rolling too far forward in the socket he'd likely feel it sooner than later. It also does not explain the unable to flex the hip when sitting issue either.
If you can't lift your leg past 90 degrees due to "weakness", as you mentioned sitting, sounds like the psoas isn't firing right. You can check psoas function by standing back against a wall and bringing the hip in question up to your chest with your arms then letting it go, while trying to use the hip flexors to hold it up. I believe Mike Robertson said in an article somewhere along the line that if his clients can't hold it past 15 seconds, he'll fail them.
I don't think you'll pass considering you said you could barely lift your hip off a chair. The real question then is, why not? Many many answers to this question...
Could be due just under use, which can be as easy as doing some psoas activation work and sprinting. Plenty of authors have written about various psoas activation drills, just search the site. Cressey had a good newsletter about working in sprinting drills for better hip flexor function too, which you can probably dig up somewhere.
It could also be a lot more complicated.
It's possible that the innervation to that muscle is inhibited. Anatomically speaking, this could be the result of a nerve impingement. In this case, that would most likely be the result of a herniated disc anywhere from L2-L4, but again, nothing is absolute!
Here is a bit of background information for you on the psoas:
Basically, Psoas issues can cause groin & hip pain in addition to messing up hip flexion past 90 degrees. I don't really agree with the latter half of the article, I don't think going hog wild with jamming a broom stick into your gut is going to get the psoas working properly again, but it gives plenty of background into the anatomy of the area to be useful.
At the end of the day, there are plenty of other possibilities. Any traumatic (but seemingly unrelated) incidents occur prior to the injury? Like getting hit on the head pretty hard? Without a more detailed history, it's really hard to tell.
If you still don't have any luck, you might want to see a professional. No location is listed in your profile, but if you live in Colorado (going off the bronco screen name), I can give you a good recommendation for a professional to see.