Most Helpful Guy
Yes, of course.
When you tell can vary. This isn't first date type of information but it is 100% needed data when you start to get serious. When you two (or someone else in the future if you and this guy don't get serious) start planning a long term future and/or possible marriage you each need to truly know the other person so that you can best determine if you are a good long term fit and what you are both looking for. You can't hide important things because they may mess things up. If you get to a long term point with someone you EACH deserve the truth so you can make an informed decision about each other and not be tricked into any relationship under false pretenses.
Most Helpful Girl
I'd say it depends on whether you want him to know or not. If you're in a dedicated relationship you're not gonna be going after other people regardless of gender. It should have no bearing on your relationship, and create no obligation to tell him, in my opinion. I wouldn't encourage lying about it if the topic ever comes up, but other than that...