It is common for young children at your sons age to be writing at a slow speed, so don't worry, I kept 5 students in today (Same age as your son) for not finishing their work, they seem to talk more than work, if you are worried about your sons education sometimes you can get him to recieve extra support, speaking with the school may help you with that, but I wouldn't worry so much if I was you, but I reccomend you talk to teachers about getting him extra suupport if he is struggling.

Sometimes young children find it scary going up a year so they would react in a way that people around them are noticing them. Does your son have any learning dissabilities? I guess you won't realise until he is atleast 10.