best friend confessing he is in love with

Ok so back story way back when i met him(my guy best friend) I was underage and had a huge crush on him. he wouldn't do anything with me being underage and I thought he was into my cousin because he seemed more into her(she was same age as me). anyways I got over him during that time and turned 18 we were still friends and he tried to be with me I wasn't into but we remained friends he is one of my best friends and we both went on to have long term relationships mine lasting longer than his which im currently still in. well now after 10 years of friendship with this guy he has confessed that he has always been in love me and wants to be with me. now I dont want to lose his friendship as its special to me it's a one of kind friendship I have never had with anyone else very intellectual type of friendship where we challenge each other's minds and make each other see differing point of views within our debates. now that he has said this I feel I should cut off ties with him to respect my SO but he is such a good friend that I'm having difficulty doing so. I kinda always knew he was into me but shrugged it off and he has never caused any issues between my SO and I he has always respected that I'm with him. he said he just had to tell me and wasn't expecting anything from it because im with my SO and have been for close to a decade now but need to be honest with me and he wouldn't let that ruin our friendship. But I feel that kinda of ruined it because I have a responsibility to my SO to keep out anyone that could cause problems in relationship. what do i do?