In the first place why are u his best friend and second of all if hes dating someone else its because he is probably trying to get over you but i mean at the end they are all fu** boys who trying to play with girls heart
Well, I feel like you should tell him but try not to make it awkward so you guys don't mess up your friendship and tell him that you still love him and you would support with everything he does and you will be there for him. Be happy for him and the person he is with if you are there for him and happy for him he would feel like he value you and I don't know he might just get back together with you
