Need some advice

So me and my boyfriend broke up last week because it wasn’t working for us… At the beginning of the relationship I made him promise we would be friends if we broke up… That was about six months ago… now that we are broken up I don’t want to be friends... I have only one class with him and I have friends I hang out with in that class... I have not told him yet that I don’t want to be friends.... and I would really like to... however he is suicidal and is just getting over self harm... he doesn’t have any other friends and I don’t want to hurt him.... how can I nicely tell him I don’t want to be friends?