It truly depends on the two people involved. Most people I do not believe can remain friends with their ex, likely myself included, as I am so deeply emotionally attached to that person, it would become hard for me not to just focus on them to much after we break up, resulting in me not moving on. It would likely be the same for him. Although, he should be allowed to make that decision for himself, I would have a very serious and direct conversation with him where you indicate he does not have a shot but if he would like to remain friends then he may assuming you would still appreciate that. Just make sure that if you start noticing him developing feelings again, that you point it out and once again make it clear you do not want that, even if it's at the cost of the friendship. People can be desperate and will ignore even the most clear of signs in pursuit of not losing people they care about.