Bad behaviour rubbin off on own child

Have recently taken on a 4 year old child who is currently on restricted school hours due to not coping in larger groups and having had problems with attention and aggression towards teacher on occasion.
Most days he is fine (better in small environments) but I have started to notice that his negative behaviour is starting to 'rub off' on my 2 year old son. Don't get me wrong my son has his moments like all kids but has never been overly naughty, I appreciate his age is a factor but was just seeking some advice as to how others may deal with this?

Thanks in advance