When it comes to suicidality, or anything that threatens the life of himself or others, take it very seriously. Error on the side of doing too much.
I suggest that you call a suicide crisis line about any concerning texts to get some professional feedback on how concerned to be, and what to do about it.
For him, I would not send more texts than normal, but I would be honest in your response, the texts that you do send, to tell him you are concerned for him, scared he won't stay alive. Even if you don't know how to interpret it - they can help figure it out with you.
If it is sucidial in nature, even if it is just side remarks or thoughts, take it serious and get the professionals involved to help. Also keep reaching out for help for you - as the threat of sudden death of a loved on can begin to trigger symptoms for others.
I told a friend once about being suicidial and when she told me how scared she was for me, it gave me the courage to go get help. It was not a burden, it helped me. I made the commitment to her for her sake to call the therapist I had then, and frankly, I don't know that I would have done that if she had not shared with me how totally scared she was. It saved my life that I finally told therapist.