It depends on the situation honestly and how close you are to him and if he’s just “talking” to this girl or if hes been in a relationship with this girl for a while. If he’s in “talking” terms with this girl and they’re just getting to know each other, it wouldn’t be wrong to tell him that you like him, but the other girl may feel as if you’re being rude and trying to take him away from you. Girls are very territorial, even when it comes to guys who aren’t technically theirs, so she may see you as someone who starting to cause problems. If this is the situation, I don’t see anything wrong with you telling him that you like him and trying to gain his attention, but be prepared to have some drama, depending on the type of girl she is of course
If his situation is situation number 2 and he’s been in a relationship for a while, I think the best thing to do in these situations is to let them be and hold onto your feelings. Sometimes the hardest thing for someone to do is to be on a spectator on the side lines. To watch someone you like be happy with someone else isn’t easy, but at the same time if you really liked that person, you wouldn’t want to hurt what they have. Sometimes the hardest thing to do is to keep your feelings inside but sometimes it’s always the best thing to do. If you were in a relationship and you were happy, would you appreciate some other girl coming in and telling him that he liked him?
If you two are meant to be, someone where down the road you two will find each other. It’s funny how things like that work, but who’s to say that 1 year from now, 2 years from, 5 years from now you guys don’t see each other in the street or find each other online, spark up a conversation and begin seeing each other. You don’t know how long they will last, and although I do not wish bad on any relationship, the truth is people break up, meet new people, find new love, etc, etc. So don’t give up, because maybe, just maybe one day he’ll notice you, noticing him and he’ll end up liking you too. It’s not impossible, if it’s meant to be, it will be. <3