The sentence suggests that because he is a kid it leads him to blow things out of proportion.
'Blow' in this context means to inflate, distend or swell.
To blow something out of proprotion means to significantly overreact to something in some way or another. In this example it means that he often reacts in a manner which the event (or whatever) doesn't warrant. In other words his reaction is not proportional to the event.
For example if he hurt his finger the normal reaction may be to say "ouch!" but if he screamed loudly and started to cry it could be said that he was blowing it out of proportion.