I would definitly suspect encopresis as this child is showing all the signs. My almost 8 year old son is struggling with this and he never notices when he smells. I would do some research on this and talk to the mother. This is a problem that most people do not talk about and may not even realize that it is a medical problem, and just punish the child for soiling in his pants when it is something they cannot control.
Please check out the website about encopresis as it has lots of info and a great active forum with lots of personal info from those who are dealing with this. Here is the link:
http://www.aboutencopresis.com/
Thank you for being a caring teacher and asking questions and not just jumping to conclusions and accusing him of being dirty. I would also suggest that you encourage him to use the bathroom after lunch and tell him to take as long as he needs as timed sits 10 min after every meal are a big part of the treatment. If you have another bathroom that is not in the classroom where he could have more privacy it may also help him to relax and have a proper BM, rather than be worried that someone may hear him or smell him when he is going.
Mel