They write about this situation a lot in songs and movies. Guy or girl is in a relationship but is way more attracted to his best friend. For whatever reason they never ended up together and one of you finds a relationship. Now the pressure between the two of you is off allowing a powerful attraction to take root.
A few ugly facts to keep in mind here. The more clearly you are able to see these facts, the better the course of action you take is going to be.
With these things in mind, you can imagine what’s probably going to happen if you pursue this. He’s in all likelihood not going to be the love of your life, and the pattern of his current relationship is just going to continue with you. If this is acceptable to you then I’d say go for it, after all it’s not going to be your relationship you’re destroying. After all, it might not go that way and you guys might actually be able to make it work.
If you really want to pursue it, you want to escape “third wheel” status as quickly as possible. You don’t want someone who is dividing his time between two females. So deliver the ultimatum that he dump his current girl quickly, at the very least before you let him into your pants.