Freelance contract

You would have to sue him for the money: that is the only way to make someone honor a contract/agreement or pay an invoice. The following is based on U.S. law--no opinion is offered as to legal options under Serbian law.
In the U.S., you would have to sue him in the county in which he resides and/or does business: U.S. courts only have jurisdiction over defendants within the geographic confines or area they cover. You also have to, unless the case settles early or easily, plan on at least one trip to that court, possibly more: U.S. courts require trial evidence to be given in person by individuals with personal knowledge, who can be cross examined by the other side--therefore, at a minimum, even if you had an attorney, you'd have to show to testify for trial, if there was a trial.
You'd have to foot or bear your own attorney costs, unless the contract stated that if you have to sue, he is responsible for your legal fees: the U.S. rule is that barring a contractual agreement to pay for a lawyer, each party pays its own legal expenses (there are a few statutes, such as those for consummer fraud or employment discrimination, that allow recovery of legal fees even without a contract, but those do not apply here).
Even if you win the case, if the client is insolvent or simply intransigent and doesn't honor the judgment in your favor, you'd have to go through collections efforts, which will cost more money (e.g. seeking orders to garnish wages or accounts receivable, to levey on bank accounts, to put a lien on real estate, etc.). And in the event of a totally insolvent defendant, or one who was an LLC or corporation which can be shut down, there is still no guaranty you will be paid.
In short, unless a very great deal of money is involved, it may not be worth suing.