He's a man I work with. (Does it mean that the man is my boss?) How can I change this sentence meaning that he is my boss.
He is not your boss unless he supervises you. In that case, say he is your boss or your supervisor.
He's a man I work with. (Does it mean also a colleague you work with?)
Your boss is the person you work FOR.
someone you work with= your colleague
boss is the man you work for.
If he is in charge of the department, personnel and work, then Manager is another alternative word.
If he is merely someone with whom you work in an office, who has no authority over you, then colleague or co-worker.