I'm no legal authority, but I would suspect that this is all going to depend upon the law in his country of residence, and reciprocal laws in the Philippines. Especially laws/rulings concerning a potential minor, (not being of legal age) to sign a contract without parental consent.

You really do need to take this before legal council in his/your home country, as opposed to getting "free" advice off a forum, but this is a rather interesting situation that would be great to follow to it's conclusion... so, if you would, please keep us informed.

I'm not an attorney either, and the prevailing law in the country and state prevails. In my area a contract is not binding until both parties have provided something. An example is you sign a contract as an artist and provide tracks to song and the record company pays you or cuts it as a record. In this case both parties have provided an actual exchange of goods, services or payment. In my state a minor can not sign a contract without a parent or guardian cosigning it. If there were a dispute in the future the contract holder could always pursue a lawsuit anyway as they have a "signed agreement" and would likely have to be "bought off" just to avoid legal fees. This is far too common in our litigous society where people recieve money just to go away as a cheaper solution than a prolonged court case.

I my country, a valid contract can only be signed by you a person that is 18 years or older.

If you are not 18 or older at the time you signed the contract is not valid.