score:5
The samenlevingscontract is indeed the evidence of your relationship.
If you want additional evidence, the most useful thing would be anything that ties his residence card to you. If the card itself mentions your name, great. If not, a copy of the application materials showing that you are the EU citizen on whom the application was based should serve that purpose.
In practice, it seems from anecdotal evidence, you are not particularly likely to be asked for documentary evidence of your relationship, but you will certainly want to have it on hand just in case.