That's a really tough question because these things tend to not go well at all because the other person feels found out, confronted.
I'd personally start out with that you love him very much and that you have something you've discovered that you want to discuss but that you will, no matter what, care for him and will respect his autonomy. This will probably cue some negative response because he does have something to hide and the thing self-injurers who are not 'out' hate the most is having the people they love confront them, however lovingly.
Don't smother him, don't punish him. Say you're always willing and reading to talk to him and give him support but that you understand if he'd rather talk to a therapist at first (that is, if therapy is feasible for your financial situation, there's sliding scale fees if insurance is an issue). It might be tempting to institute body checks and invading privacy but that will likely make you seem the enemy rather than somebody who is on his side. That is not to say, that if you feel he is in grave danger and needs emergency mental health or health treatment that you should stand by. If he's a danger to himself (suicide or severe self-harm that puts him at risk of disabling or killing himself) then, of course, hospitalization should be considered.
Good luck and I hope it goes well.