I found that you could marry if at least one of the couple must be resident in Belgium. It sounds like your fiance fits that standard.

Documentation and Requirements for Marriage
All documentation needs to be taken to the marriage office in the commune where at least one of the couple lives. This office will be part of the Service de I'Etat Civil/de Burgerlijke Stand and can be found in the municipal buildings of the Town Hall (Maison Communale/Gemeentehuis).

Original or certified copies (not photocopies) of the following documents are required:

Birth certificates for both parties
Certificate of residence, showing full name, place and date of birth, last legal residence, nationality and marital status for both parties. A person who is not a Belgian resident must obtain their certificate from the authorities in their last place of residence. If this is not possible (the US authorities do not issue such documents, for example) then ask the local Belgian authorities what they will accept instead
If either party has previously been married they must provide a copy of the final divorce decree (or death certificate of the former spouse if widowed). Any divorce decree must show the date on which the divorce became final. If not, the court that pronounced the divorce must provide a statement giving this additional information
Once the documents have been delivered, the relevant registrar in Belgium draws up the official copy of the marriage notice form (acte de déclaration de marriage/akte van aangifte van het huwelijk).

Certificates and certified copies can often be obtained via the relevant embassy in Belgium. All official documents issued in a country other than Belgium need to be authenticated and if they are written in a language other than Dutch, French or German, they must be accompanied by a translation from an approved translator. If the translation is not undertaken in Belgium, the translator's signature will require authentication.

Non-Belgians marrying a Belgian citizen do not automatically become Belgian nationals themselves, although they can apply for Belgian nationality following the marriage.

For the more complex details, you should probably contact your embassy in Belgium (http://www.indembassy.be/).

Congratulations!

 

Unfortunatley I don't have personal experience with this. I would recommend contacting your embassy about your particular case and they can help you further. Or maybe someone else will have some more advice. Best of luck!