Someone that "athletic" + skinny. It wouldn't take much at all, infact 6 beers within 3 hours would bring him to a 3-5 range on your scale. If he added in a few more within that time it would exponentially increase. This is my mere opinion on how drunk he was, but I believe he was around a 3.5-4 based on the ammount of beer bottles, the idea of the bottle size, and his weight.
There are several other factors to consider as well:
1) Did he eat before drinking?
2) Did he drink any water or other beverages?
3) Did he recently do anything to increase his BPM (Heartbeat)? (Ex. Jogging)
4) Was he warm or cold, etc
With each variance in the situation - comes a different rate on this "1-10" scale.
Factors of why he didn't seem that drunk could be:
1) Cold showers
- Anything to increase how "alert he was" without actually sobering him up.
Assuming he's an adult I doubt there would be much difference at all. Six beers isn't really that much. I can drink six beers and be fine and I'm a small girl. Men also have a higher tolerance for alcohol in general. So I would assume six beers would have put him at a 2-3? I'm just curious why you are so concerned with his drinking though, lol.