Can you please, help me with the following:

Brad was a kind of mascot in the squad, and the fact that he still had most of his baby fat gave him a round, cartoonish appearance. His thin blond hair always looked as if someone had just rubbed a baloon against it.

To rub a baloon against sb's hair- is this an idom, an expression in English?
I guess it means that he was still young and immature?
Hi again, Antonia!
I think it just means his hair had a spikey look, stood up on its end.
As far as I know, it isn't an idiom, just a comparison.
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I agree with pieanne: it's a simile, rather than an idiom. When you rub a balloon against your hair, static electricity is transferred to it, and so it stands up on end - Brad's hair is unkempt and it sticks up!
Oh God! I was so stupid! Of course.
A friend of mine tells me they used to rub ballons against their hair when they were kids. Funny, I never did that:)