I was a depressed teenager resistant to help.
I think the reason why the helplines advise u to focus on ur mh is because if u feel wholly responsible for u son, and act as if he is harming Ur or ruining Ur life, it will make him feel worse.
He needs to know that u have Ur own boundaries and ways of caring for urself, which can role model the same to him.
I think accepting and being open with him that u are not sure urself how to help, but that u love him and believe he will get well, and see past his depression, into his essence and his good qualities, is a helpful approach.
U can't make him get help. But u can listen and have curiosity about how he is..,,and if he has someone to listen he may eventually,through having a safe space to explore his feelings, come to seek help himself,
The power of a no judgemental person to listen cannot be understated.
If Ur interested, I'd be happy to recommend some memoirs about depressed young ppl and how they found a way out....it may help to see that it is often indirect support and love which was the very thing that helped the person back on their feet.