You say teen, so I would have to say no, he's not fat.  He will do a bit more growing yet, so I wouldn't worry about him.

I went onto this site.. (well found one) and for the height and weight of your son.. he's fine.. he's in the normal BMI weight chart. http://www.nhlbisupport.com/bmi/
There's the site that I used.

I'm over weight.. All kids go through it.... Everyone's goes up and down all the time.
If he's eating fine, and doing everything like that, don't worry about him.. Unless he says something. Or take him to the doctor!!

how old is he? my son has gone through different stages when he had a little belly and then he grew again and the belly is gone but my son is only 14.

Sounds like an okay weight to height ratio to me. Could be he's letting his stomach muscles sag rather than holding them taught? Easiest way to tell is to do a pinch test. If he can pinch more than an inch of fat on his belly then he might want to watch what he eats or get a bit more active.