Potential Danger here...

Is the guy typed for single pilot? If so, he can take you along and give you dual.

If he is only typed for dual-pilot ops then an SIC is required, and YOU require additional training before serving as SIC on the aircraft. This is specified in part 91, and is not too hard (or expensive) but still needs to be done and logged.

Note: It's OK to log several of your first flights in an aircraft as dual-received, that's legit training. But DO NOT fly 100 trips with this guy over the next year and log it ALL is dual. The FAA does not allow this, log the first 2-4 flights as dual, then after that you should be the SIC.