Nearly any amount of marijuana could be considered a usable amount by the Court, but it is an issue that can be litigated.
Miranda warnings only need to be given in a situation of custodial detention. In short, did the child believe that he was free to leave or to not answer questions. In J.D.B. v. North Carolina the Supreme Court ruled that children should be treated differently in regards to Miranda and that their age can be a factor in determining if they felt they were in custodial detention.
If he was held for two hours without contact with a parent and asked incriminating questions there may be grounds to suppress his statements.