Lots of communication. Discussing everything up frlnt. What do you need in your relationship and how will you be sure that your needs are still being maintained while he has multiple relationships. Like how many days q week will he be going out. How will you feel and do while he's away. What are the expectations of sleeping elsewhere. What aboutbhis partners sleeping at your house. How will he make sure the needs of his other partner are being net if they conflict with your needs. What if you hate his other partner? Is he going to be expected to dump them or how will you deal with that? What about birth control? What if his partner wants to go bareback or have children with him? Maybe someday wants him to live there part-time or even move in with him. There really is a ton of things to discuss and it really should.
If he meets someone and is experiencing he euphoria of a new relationship, things can get crazy, esp if boundaries haven't been set first. When my ex met his first gf we ended up separating and nearly got a divorce because we were just wing it.