He has never seen me play and I have never seen him play. Should I be worried that he is really good or should I expect him to be really bad?
He's either a jerk looking for free money or he thinks you two are both about the same level and wants to win money. Or he just like MMs for whatever reason.
Very few people who do MMs are bad. The few I know who do so usually do so with people they met at tournaments who they became friends with but don't see each other outside of tournament, so they do MMs with those people where the games-to-payout ratio is high (for instance, one guy I knew often asked people for 1 dollar MMs where it was first to 10 games. He usually got 10-0'd or lose like 10-1 or 10-2, but people would play him seriously and he'd get a lot of practice and get advice afterwards, and he felt improved this way - it started to show after some time too). The other time I see MMs is with unique rules (for instance, I recently did [and lost] a $5 MM with the number 2 guy on the PM PR - but our rule was Big Blue only, and though it got shortened to Bo3 from Bo7 (so that it could be onstream), I did get a game off him).
I hope that the dollar value is relatively low (that is, I hope you only put like $1-5 on the line, or maybe $10 tops). I wouldn't do money matches with people I hadn't seen play for more than $5 tops, and I'd make it a Bo5 so that if they are way better than me, I at least get some value out of my money by getting 3 full games.
If you lose, I highly recommend NOT re-challenging him - if it was close, he could've been sandbagging in the hopes you'd ask for a rematch to get more money. Some people out there are scum like that. Even if he's not, no use risking it until you've seen him play others (several times, since he could sandbag a few times but it's unlikely he could sustain it).