Maybe this is a dumb question, but how will we know if you choose not to post a losing bet? It seems to me that you could win 4 in a row, then loose one, but don't say anything about it, and then win 3 more and claim that all 7 were "in a row" even though there was a loss in the middle. I'm not accusing you of doing this, I just don't understand how we would know if you did do this.
The only way would be to announce now which games he will bet on and only release the specifics right after the match starts.
Other than that, it would indeed be impossible and we would have to take his word for it.