Friendship can develop out of relationships, you just have to make sure that it is the same feeling both ways. You two were a big part of each other's lives, so it is not a big deal that you have some things in common.
If you think he may want more, just have a talk with him and let him know that you don't want this to be awkward for him. Let him know that you want to be friends, but you don't want him to have any issues with it. After that conversation, any problems he has are his and there is no need for you to worry.
Good luck.
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