Yea - you're in dangerous waters - but suspect you don't need anyone to tell you that
The bad part of this is that often these situations are the ones all the horror stories are built from that discredit the whole concept of poly relationships. If he get's caught and she's a nasty type - it can get down right dangerous !
My thoughts are that you two have connected and there MAY be real potential. Kind of like a testing process. Test is basically over at this point.
If it were me - and this is just me - I'd have the discussion with him about "potential" but explain that currently the risks are simply not worth the rewards for all of you. You've tested the water with minimal damage so far. He needs to go back home and make some decisions for himself on what his real relationships is - and can be - at home. Until he has that sorted out, as hard as it probably will be, I'd keep my distance.
In living a lovestyle like this unfortunately we seem to run into situations that show "promise". Reality & practicality often overrule.