Are you sure it's slow "because of the amount of crap on it"? Could there be some virus or malware slowing it down?
VHG's reply is absolutely correct and this is something your son should be able to do easily.
Can you give us some details of his laptop please? (Make, model, spec, age, operating system, capacity of hard drive, percentage of hard drive still available)
Does your son actually know anything about computers or does he just want something bigger and better?
If your son is really into computers he should either know all these things or be able to find the answers very quickly.