By being very clear and direct with him. Then cut off all contact *and* stick to your boundaries.

It seems the message he is getting from you is ambiguous i.e. you don't want him around but felt sorry and reached out to him when his dad died... you don't want to be with him yet you are dating him.

If you don't want to be with him, cut off all contact and stop reaching out to him. Any ambiguity felt will be further signal for him to keep pushing. He clearly does not seem to respect you / your boundaries anyway, so do not feed him any more mixed messages.