I know you're talking about getting into another relationship, but poly is a two-way process. It sounds as though you're going to have the most issues with him having another partner, and you're hoping to mask that bad feeling by getting someone yourself.
The problem with this is that you still have that bad feeling, deep down. I'd say it would be better to let your partner start CASUALLY dating (but no further for now). Then explore the feelings, make sure you're getting all you need and start feeling your way through things, working your way up to the relationship you both want together. I trust that you know (or are working towards knowing) what that is...
I don't think there's many people who find it most difficult to be with someone else, since we all have multiple relationships throughout our lives (even if not all at once). I think the true test is how you react to your partner having someone else - there's a lot of feelings that get unearthed when your partner is out with someone else.
Given that he's poly, and you're not sure, I think that the best idea is to get used to the idea of him with other people, and don't worry about finding someone else yourself. If you do find someone, then fantastic! But I think it's going to be tougher long-run if you're not dealing with these emotions, and are instead surpressing them.
Best of luck whichever you choose