The best thing for you to do now is to sit down and write or type everything you want to say in an email explaining why things were so awkward at the convention and explain why they were on your side and tell him that you want nothing from him if only to be his friends and that you don't want things to be so awkward when the two of you see each other. Explain to him that you think he is funny, nice and an overall good guy you would like for him to be your friend and that you don't expect anything else from him maybe if he knows that you aren't trying to get a relationship going with him he may be more able to have a friendship with you but one thing the both of you are going to have to do is stop allowing your friends to be involved with your friendship or relationship they may think they have your best interest at heart but are doing more harming then helping.I would write the email and then ask if the two of you could go to lunch as friends just to talk things over. You may not have any other choice but to talk everything out in order for all the hurt feelings to be put on the table and dealt with.
Other than letting the whole situation go and chalking up to a learned experience writing an email to him is your best bet at this point or let it go. Explain what happened at the convention would not be beating a dead horse it will be explain to him what happened at least on your part. Things are always going to be awkward with the two of you until you nip this in the bud. You don't have to go into details about the past when the two of you were dating just explain that you felt uncomfortable just walking up to him and talking to him and that you had hoped that he would at least so say hello as friends. What often happens in these circumstances is that the other thinks they know what the other is thinking and many times we can be wrong about what they are thinking or what they want from us. It would have been best since you first contacted him for you to say hello to him when you saw him at the convention. With that being said you have three options call him on the phone and talk to him about the two of you being friends, have a trusted friend of the both of you talk to him about being friends or writing an email explain to him that you want to be friends.
I would give him one to two weeks before contacting him again this way if he were upset about your ignoring or avoiding him he will have time to get over that and then you can start fresh. I would write the email after two weeks and see if he replies to it and if he does take it from there but if he doesn't don't get discouraged that may mean he needs more time.