Hmmmmmmmm...
Lets start with the easiest thing first... the hair...
I would probably try and put a net in the water and bring him near the surface (but not out of the tanks water) and maybe by hand very carefully remove the hair, or if need be, maybe use a pair of scissors to carefully cut it off the puffer...
After doing this i would check to see if hes perked up at all, as maybe that strand of hair is annoying him??? Not that i can say i can clearly see it in the photos...
Now if that doesnt work, then we need to work out why your puffer maybe acting funny, and a few questions:
1. did you ever do a round of dosing for IP's??? If you have, do you find his belly still flattens extremely fast after big meals (say 24 hours after a large meal)
2. Whats your feeding schedule and what are you feeding him? Any hardshell foods?
3. Is their enough aeration in the tank either through breaking surface tension or an airstone?
4. He looks to be a young puffer, how long have you had him and how is his growth rate so far?
5. What does your puffers poop look like? Is it stringy or more like pellets / small tiny capsules
If you can answer those questions and let us know water paramters etc, we can try to narrow down possible causes???