Hello, I'd like to help you with your question.
Getting someone to get treatment when they refuse is always difficult, especially when dealing with an adult that you have no control over. You want them to see the importance of taking care of themselves but for some reason, they refuse. It can be upsetting when you care about the person and they will not listen.
It may help to ask your nephew what he might be willing to do instead. If you can, talk to him again and let him know you are concerned about him. Ask him how you can help.
He may be doing what he is doing because he feels these people care about him when his parents did
not. His pain from the abuse and neglect
might be very deep right now and he is seeking out others he can identify with and who seem to care about him, something he may feel he never had. So it may help to approach him with that in mind. You have told him that you love him and miss him, which is a good thing to do. It helps him to know that you are there, in case the situation he is in breaks down and he needs somewhere to go. Keep doing that, and also add
that you would like to see him more often. Then try to keep building a relationship with him through doing things together. See if he will go with you to dinner or to an event. The more he can relate to you and have experiences with you, the more you can bond and have more influence in his life.
Here are some other resources to help you with ideas on how to help your nephew (though he does not seem to have a mental health issue right now, these will still help):
I Am Not Sick I Don't Need Help: How to Help Someone with Mental Illness Accept Treatment-Xavier Amador.
Most of all, offer your support. Be there as much as you can for your nephew. Sometimes that is what makes the most difference in helping him to change.
If you have tried all of the above and he still refuses, you may have to let it go. Your nephew needs to make his own choices, even if they are not the best ones. And you do not want to create more tension in your relationship by making this a big issue between you. You want to try to keep your relationship on good terms so you can keep an eye on him. And if things ever change and you can try again, maybe he will listen.
I hope this has helped you,